We meet Tim Bruce-Dick outside the entrance to St Pancras station. He's an architect and tour guide and is not difficult to spot - straw hat, red shirt, blue linen jacket and a general air of beatific bemusement.

Tim has arranged a weekly architectural tour every Wednesday at 6pm, somewhere in London, over a 14-week period for a very reasonable £7 (plus £5 for a friend). And tonight he is covering King's Cross and St Pancras. "I've had 40 phone calls," he says hopefully.

By 6.10pm, a dozen of us have turned up, but if genuine curiousity were a measure of quality, quantity didn't matter. The development of St Pancras station and the land behind King's Cross was what drew most of us - in the event, we never get around to King's Cross as St Pancras and environs take more than the allotted two hours. Thanks to Tim, it was quality time.

He hands out a street map with the most intrically hand-drawn notes in the margins, from the tower blocks of the Goldington Street Estate (designed by the St Pancras Borough Surveryors Department, 1949) to Kings Cross station (Cubitt, 1851) and the new St Pancras International (Foster, 2007).

He comments on plans for a competition to design a new frontage for King's Cross and expresses a hope that it doesn't involve too much foliage. "It's one of my architectural bete-noirs, putting trees in front of buildings," he twinkles. "You wouldn't get Italians putting trees in front of St Peter's, would you?"

It's to the new St Pancras station we head first, under the care of Laura Wise from London & Continental Railways, owners of the station. The original station, which opened in 1868, took 6,000 men and 100 steam cranes four years to build but in recent years had become a darkened diesel fume-filled cavern. Now a spectacular renovation is nearing completion, in time for the launch of Eurostar services to the continent in November.

There are 14,800 panes of glass in the renovated roof: at sunrise and sunset, the slanted rays should create a fantastic lighting effect. Laura takes us to a viewing platform (any group can arrange a visit - email her at lwise@lchrq.co.uk) from which the huge cathedral-scale of the peacock blue girders and red brickwork can be more readily appreciated. The roof is 100ft high and 700ft long - hence the new Foster extension to accommodate the Eurostar trains which stretch for quarter of a mile.

In all, there are 10 platforms and when the new international terminal is opened, it will become one of the largest transport hubs in Europe. Around 25 million passengers are predicted to use the station every year.

L&C hopes many will be visitors, not passengers, attracted to the longest champagne bar in Europe and shopping. In 2009, many will also be staying in the old Midland Hotel, the red-brick buildings in high Gothic style which fronts the station and which was designed as a railway hotel by Sir George Gilbert Scott.

The last paying guest left more than 70 years ago, and the hotel was only saved from demolition in the 1960s by the intervention of Sir John Betjeman, who helped the building gain listed status in 1966. Now a hotel will open in February, 2009, and 67 apartments will be added at the top of the building. The hotel will boast a ballroom and swimming pool. Several rooms and the main restaurant will be returned to the original Victorian style and many original features, including a cantilevered staircase, wrought ironwork, stained glass and Minton tiling will be restored.

From the freneticism of the stations and Marylebone Road, we turn into the quiet of Midland Road, on the far side of St Pancras station. Tim points out Scott's pointed windows which characterised the Gothic style and which, in the mid 1850s, rivalled the Roman classicism of King's Cross.

I'd read about Tim's tours in the Camden New Journal, in which he expounded his architectural beliefs. "Each walk attempts to take up contemporary architecture but there are so many buildings you can't ignore - such as the concrete architecture of the 60s and 70s. You can't ignore the past.

"At the moment, the trend is high-tech. This has really triumphed in the last 15 years and defeated post-modernism. That has really sunk beneath the waves."

He doesn't over-elaborate, as is his way, but moves swiftly onto the British Library, where he opines: "The inside is more successful than the outside." The library was built by Professor Sir Colin St John Wilson - who died this week, aged 85. It took 25 years for his project to be built amid constant vilification by, among others, the Prince of Wales who described it looking like "an academy for secret policemen."

In his obituary in The Times, Wilson said he suffered from the criticism: “As a result of being the architect of what has been called the great British disaster I have no work and my practice, the actual partnership, has now dissolved,” he said as the library opened its doors in 1997. Tim shows professional sympathy: "Architects hate a building after 25 years, but come around after 50 years. The library is at that changing stage..."

Out of the library and down Ossulston Street, we see remnants of the original 12ft high brick wall that surrounded the old goods yard on which the library now stands. A favourite of Tim's are the council blocks at Cecil Rhodes House on Goldington Street, which made striking use of vacuumed glass brieze blocks 50 years before they became fashionable again.

Across the road is St Pancras Old Church, where a mausoleum for Sir John Soane's wife is said to have inspired the design of the telephone box, and where The Hardy tree acknowledges where the young poet worked as an architect's clerk in exhuming remains during the building of the Midland Railway in the 1860s.

Back towards St Pancras, via the coroner's court, Camley Street wildlife trust and the old gasholders we passsed 8pm - and had yet to start on the architectural history of York Way and environs. Another day perhaps, and another tour.

Tim can also be reached on 020-7485 8976