A Diesel engine technology has advanced in leaps and bounds in the past 10 years, producing more power with fewer emissions and better mpg. This is thanks largely to closer build tolerances and, as a result, diesel engines rely more than ever on the lubricating qualities of diesel fuel, which is thicker and more oily than petrol.

The more the petrol has been diluted, the less damage it is likely to cause, so if you were just topping up the 60-litre tank, you are less likely to have a problem than if you’d run almost dry and put in £10 of fuel to get you home.

We’ve heard of drivers who have got away with simply filling the tank to the brim with diesel, but it is risky. It’s possible you’ll cause damage to the fuel pump or other parts of the fuel system and if your car is under warranty any claim for failure of relevant parts would probably be refused.

Ideally, the tank should be drained from underneath and the whole fuel system, from tank to engine, purged of petrol. That really makes it a garage job. A Peugeot dealer will be able to do this for about £200. To avoid a repeat performance, we’d suggest investing £14.96 (including postage) in a Dieselguard (0798 0516 848, www.dieselguard.com). This is a small electronic box that sticks to the inside of the filler flap and speaks a warning every time you fill up.

Q My car is a 4 litre 1995 Jaguar XJS Celebration, in tip-top condition, bought for and still valued at £10,700. Until recently it had a full main dealer service record but the record was lost by a repairer. I am now planning to sell the car. How much will I lose because the car no longer has its service record? — MG from Surrey

A Older, specialist cars such as your Jaguar are never easy to value because the “book prices” tend not to reflect what a keen buyer would pay for an immaculate example. The book value for an XJS Celebration with 90,000 miles on the clock is about £6,500 for the cabriolet or £5,500 for the coupé; a 60,000-mile example would fetch about £1,000 extra.

However, you will find that some insurance companies will agree a higher replacement valuation (called an agreed value) than the book price because they know the cost of replacement may be higher.

While £10,700 sounds an optimistic figure for the car it may be achievable if you find another buyer like yourself who is keen to own it.

The loss of the service history may put some enthusiasts off and we would estimate that if two identical XJS Celebrations were sold side by side, the car with the service history would command about £2,000 more than the car without.

Q I have seen spray-on products advertised that claim they are invisible when used on car numberplates but prevent speed cameras reading the registration. Do they work and are they legal? — BM from London

A Despite claims made by the manufacturers of these products, the Road Vehicles (Display of Registration Marks) Regulations 2001 states that a numberplate must not be treated in any way that makes the characters less distinguishable to the eye or that would “prevent or impair” a photograph.

Though the chance of prosecution may be slight, technology could also get the better of you. Many of the cameras operated by the police and Driver and Vehicle Licensing Agency no longer rely on flash photography but use digital imagery and infrared to record the characters and would not be affected by a spray of this kind.

Q We are expecting twins next month, and also have a daughter who is three. We are having difficulty fitting three car seats across the back. I have a three-year-old Peugeot 307 estate and wonder if there is a solution. Will we need to buy a new car? If so, what would you suggest? Including the trade-in on my car (which has done 60,000 miles), we want to spend no more than £10,000 — NF from Royston, Hertfordshire

A Multi-activity vehicles such as the Toyota Corolla Verso, Seat Altea and Ford Focus C-Max may be your best bet. The cars are generally no longer than a standard family hatchback but, being a touch wider, have three distinct seats in the rear.

All three cars score an impressive four stars (out of five) for child occupancy safety, while the Toyota and Altea score five stars for adult occupancy safety (four stars for the Ford) in the Euro NCAP tests. A 1.6 VVT-i Corolla registered 2004 04 with 20,000 miles will cost about £9,500 (a diesel model will set you back about £1,500 more) and a 1.9 litre TDi Seat Altea Stylance of the same age can be yours for a touch over £10,000.

The Ford Focus C-Max is cheapest of the three and an equivalent age 1.6 TDCi Zetec will be about £9,500. Expect about £5,250 in part exchange for your Peugeot, assuming it is a 90bhp version, or about £5,500 if it is a 110bhp model.

Q I have a classic car that is kept covered on my driveway during the winter. It has two covers (breathable and waterproof) and I also leave silica gel inside to reduce moisture. Despite my efforts, this winter the car has developed a serious mildew/ mould problem on seats, armrests and roof lining. How can I stop this recurring next year? — DH from Lancashire

A First off, you’ll have to clean the seats, trim and headlining using a quality cleaner such as Turtle Wax Interior 1 which costs £4.99 for 500ml. For leather trim and seats we’d recommend Gliptone’s leather cleaner, priced £5.95, and leather conditioner, £6.95 (01706 819 365, www.liquidleather.com). If you’ve got a domestic dehumidifier, try putting it in the car to draw out excess moisture — but use a residual current detector for safety and don’t leave it unattended.

Next winter, try removing the carpets and keeping them in the house, which will protect them and also reduce the chance of trapped moisture corroding the steel floor. It might sound extreme, but taking the seats out is also recommended and fairly simple in most older cars. If you keep the car windows slightly open (except when it’s raining), the air circulation will help to dry the interior. This is usually impossible for security reasons, but if you can do it at weekends when you can keep an eye on the car you will see the benefit.

The ultimate answer may be the Carcoon (0161 737 9690, www.carcoon.co.uk). This is a sealed transparent plastic bubble that encloses the car in a double-skin construction with a gap between the two that traps air to provide insulation. The outer layer cuts UV rays, which can damage paintwork and rubber seals (cracked seals lead to water ingress which makes the interior damp and mould problems worse). It also reduces infrared rays to help keep the interior temperature constant.

Two small electric motors circulate dry air within the bubble 24 hours a day. Prices range from £439.16 to £571.34 plus delivery, depending on the size of the vehicle, and it should only cost a few pence a day to run.

The Permabag (JF Stanley & Co, 00800 2222 2233, www.permabag.com) is a similar product but does not need electricity. This zip-up bag wraps around the car and comes with canisters of a drying agent that go inside to lower humidity. Prices range from £361 to £710.

Q I want to buy a two or three-year-old saloon. I’m willing to pay up to £10,000 for something smart with a bit of get up and go. I have been driving a Peugeot 605 2 litre turbo that has performed well but is looking a bit tired. I am considering a Saab 9-5 or a Peugeot 607. What would you suggest? — TM from Newcastle

A A Peugeot 607 diesel could be the car for you. The 2 litre HDi engine has 26bhp more than your car and a high level of equipment. What’s more, large French diesel cars do not tend to hold their value, which makes them attractive second-hand buys. A 2002 52-plate with 30,000 miles could be yours for £9,000-£10,000.

If you fancy even more oomph and a little more prestige then a Saab 9-5 might be even more appealing. The 3 litre V6 diesel produces 176bhp and if you can find the Arc model you get leather upholstery as standard. With £10,000 you should be able to afford a 2001 51 plate with 40,000 miles from a dealer. Buy a higher mileage car from a private seller and it would probably cost considerably less — not bad for a car that cost £26,000 when new.

A final suggestion would be a Rover 75, the best car Rover was making before it shut down. A 129bhp 2.0 CDTi Connoisseur auto, registered 2004 04 with 30,000 miles, should just sneak in at the top end of your budget. It’s the newest car of the lot and you will be impressed at how well it drives.

E-mail your questions for our experts on any aspect of motoring to carclinic@sunday-times.co.uk or write to Car Clinic, Driving, The Sunday Times, 1 Pennington Street, London E98 1ST. Please give a daytime telephone number. We cannot send personal replies or deal with every letter. Please do not send original documents or SAEs. Advice is offered without legal responsibility.