Your children are suddenly adult-sized. You find you have acquired a large golden retriever. On weekend trips out of London there always seem to be three or four bags, a briefcase, a computer, two piles of homework and an improbably large radio-CD. The retriever kicks up rough at having to share his space with the luggage. So do the children. So, come to mention it, do you. In short, you need something larger.

It is, presumably, a common dilemma. Hence the boom in MPVs, van-shaped things and vaguely military-looking vehicles with huge tyres and drivers in obligatory dark glasses. Hence I am thinking about a Jeep.

Even as I put down those words I have a Mandelson moment: I can't believe I am writing this. So let me qualify those words immediately. The Jeep was the kids' idea. A suggestion that was met with a snort from me. The truth is that I have always nursed a secret Jeepo- phobia. They seemed a bit boxy, clunky and just a bit flash - or, to put it another way, naff. You picture them lining the streets of Chelsea outside the homes of junk bond dealers or Radio 1 DJs, venturing off road only to mount a pavement.

So the arrival of the test Jeep in the Guardian car park was an awkward moment. The new Grand Cherokee is not what you would call a subtle presence. It is tall (1.7m), long (4.6m), fat (2.2m) and possibly just a bit pleased with itself. It is difficult to climb up into it for the first time without a twinge of self-consciousness.

Once inside - and once you're sure no one you know is looking - it is a different story. You wanted space, and now you have it. You are high up and surrounded by glass - and if the passenger door seems a long way away, the boot is very, very distant indeed.

This is American executive-style comfort of a high order. The seats are of Club-class proportions and programmed to remember your favoured contours. There is fingertip control of more or less everything, from radio channel-seeker and volume to cruise control and accelerator. A digital display tells you in which direction you are pointing, reminds you when next to fill the tank, and prompts you to buy flowers for your PA's birthday. Well, almost.

So you set off in sedate fashion, looking over walls and down on your fellow motorists with some satisfaction. Out on the open road, the 4-litre Jeep is not without acceleration. But the back streets of London are not its natural terrain, and there is no sense in rushing things. This is not a car for ducking and weaving or nipping down to the dry cleaners. It is a car for loading up, switching on the CD and pointing down the motorway.

The kids were, naturally, delighted. The dog also seemed pleased with his living quarters. The boot of the new Jeep has managed to lose its spare wheel - now buried beneath the floor - and is as palatial as everything else about the vehicle. It was just me left worrying.

The very thing you wanted it for - its space - is the very thing that nags away except when cruising down the open road. The publicity pictures show the car among rugged mountains, lakes and rocks. Quite so. You don't see it pictured squeezing its way down an English village high street, trying to fit in an underground parking bay or doing battle with an obstreperous cabbie in a north London rat run. Its sheer size and solidity mean that you usually win, but you pick up some dirty looks on the way.

All that said, it is a handsome beast. The new Jeep is more rounded and marginally less boxy than its predecessor - but unmistakeably still a Jeep. It is said by Chrysler to be 'a completely new vehicle' with only 127 parts carried over from the previous model.

At the end of the week it was back to the anonymity, scale and lightness of touch of the regular estate car. Which, in many ways, was a relief. On the other hand, all the problems of fitting in kids, luggage and dog flooded back. There's just no pleasing some people.

Jeep Grand Cherokee 4.0 Limited £29,995; 4.7 V8 Limited £34,995.