So the sun has got his hat on, the blossom is nodding on the bough and we’re all thinking: “Hey . . . wouldn’t it be so great to have some people over?” We could throw some steaks on the barbie, invite a bunch of kids over to scream and splash in the pool (not that I have one, but stay with me) or yell and bounce on the trampoline.

Well, that was what I would be thinking if I hadn’t read about last week’s bouncy castle case. OMG, as my daughter would say. What a horrible, depressing story this is. For everybody. Let me explain.

In September 2005 a Mr and Mrs Perry decided to throw a party for their triplets’ 10th birthday party and hired a bouncy castle. In the adjoining field a boy called Sam Harris, then 11, was playing football with his dad and a bigger boy, aged 15. They asked Mrs Perry – who was supervising the inflatable – if they could have a go. At some point during their play session, Mrs Perry turned away to buckle another child into a harness for the bungee run, which she was also supervising, and the older boy did a somersault and kicked Sam in the head.

Poor Sam’s skull was fractured and he sustained severe brain damage. Last week a judge ruled that the Perrys were negligent and, as the hirers of the bouncy castle, were liable for damages for Sam’s injury. “I conclude the level of supervision of the bouncy castle was inadequate,” the judge said. Damages are expected to run into seven figures although the Perrys – tellingly – say they are reluctant to appeal. Why should they? They are not paying. The insurers are.

So does this mean that you shouldn’t hire a bouncy castle any more for a kiddies’ party? The law is quite clear. It states that if you invite people onto your property or allow them to use your stuff, you owe them a general duty of care. This was legally established many years ago.

When it comes to bouncy castles, I wouldn’t mind if I never saw one of them again; they are vile things that pong of sweaty rubber and are particularly disgusting in the rain. But love or loathe bouncy castles, this ruling is bad news for everyone – except lawyers, of course.

I decided to call Peter Cornall, head of leisure safety for the Royal Society for the Prevention of Accidents (Rospa), to see what the official line is on all of this.

I said I found the case worrying because we all have friends over and it’s hard to supervise them all the time, especially if they are bigger than about five years old and the grown-ups are drinking alcohol. I was also puzzled as to why it was the Perry family who were being held responsible, when Sam’s dad was there and had given permission for him to jump on the bouncy castle.

“If this was a car accident and the Perry parents were driving and had turned away to buckle in a child in the back seat, and at that moment had collided with the Harrises and Sam Harris had sustained his injury that way, you wouldn’t be quibbling about the fact that the Harrises were suing the Perrys to get the maximum damages for their child who needed lifelong care,” he told me.

A car accident is different, though, I said. With a bouncy castle or a trampoline it’s about using common sense and accepting that activities carry risk and accidents do happen, especially during boisterous parties and among friends.

I was thinking particularly, when I said this, of Jeanette Sykes, 39, a hairdresser who was rendered tetraplegic after she twisted and landed awkwardly on a trampoline at a friend’s barbecue, but last week happily announced that she can walk again. She concentrated her energies on getting better.

So to try to clarify the position, I asked the Rospa chap whether, if one of my children’s friends broke his neck while jumping on my trampoline, I would be liable.

“You would be,” Cornall said. “Same as if a child wandered into your garden and cut his arm off with a chain saw you’d left lying around, or drowned in your pool. No, if accidents happen you have to find someone to blame – and sue them so they [the injured party] can be assured of getting adequate care.”

As a trampoline owner, this came as a shock to me. It’s not just that I don’t relish the prospect of standing sentinel at the edge of our Super Tramp as large children fly through the air – occasionally colliding with each other – for hours on end or dutifully sweeping my outbuildings for chain saws and other potential booby traps.

No, I thought Rospa was in the game of encouraging people to take reasonable steps to prevent accidents, not egging them on to sue anyone they can. I thought that judges these days were supposed to take the principles behind the Compensation Act 2006 into account, an act that emphasised that some activities were desirable despite being risky. The judge in the bouncy castle case last week appears to be taking the duty of care principle to a ludicrous extreme that will place an impossible burden on every householder. Perhaps this is why the judge gave leave to appeal.

I thought we were supposed to be encouraging our kids to take more risks – as Ed Balls, the minister for children, has himself bewailed as he launched a kindred campaign against “compensation culture” and “cotton-wool kids”.

Yet now we have the bouncy castle case and a ruling that – if left unappealed – will mean that anybody who ever invites a guest into their house, child or adult, opens not just their homes to friends but themselves to lawsuits.

Moreover, it will give schools further reason to abstain from doing fun stuff off campus, will inhibit adults from spontaneous socialising (unless insured to the hilt) and, above all, will remove the onus on people to behave sensibly and accept responsibility and instead place it on finding someone – anyone – to blame when things go wrong.

- Speaking of things going wrong, I was a panellist on Question Time for the first time last week. It was terrifyingly traumatic. I was billed as the sister of the London mayor (mildly annoying from David Dimbleby, who resists being called Jonathan’s brother) and cast as Tory girl.

I fluffed and stumbled; a button of my dress popped open; I had as my cufflinks the big beasts Ming and Hezza (Campbell and Heseltine); and then, to top it all, Piers Morgan launched into a tirade against my brother.

Afterwards he sent me a text: “My mother thought you were squinting at me in a menacing way as I insulted Boris. I said it was a sexual tension thing.” I texted the cheeky tinker back with the news that my mother – and Boris’s – had intended to watch, but she had fallen asleep before the programme had even started. A very good thing, considering.

rachel.johnson@sunday-times.co.uk

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