Thanks everyone - a very interesting discussion. I actually wish this change had happened 20 years ago, as it would have saved me from being ripped off a couple of times when I was younger.
Anyway, the good news is that I managed to get what I think is a fair deal on a Subaru Forester. I was only nearly convinced, until the salesman let me take it up a flooded and rutted farm track: SOLD!! This is no pretend 4x4!
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Anyway, I managed to negotiate down to £400 below Parker's guide price, plus a tank of fuel and a towbar fitted at cost (yes, I was sceptical about that bit too, but two local towbar places wanted significantly more, so possibly not!). It certainly ain't a bargain, but it seems a fair price to me, and was all agreed courteously, so I'm happy. Actually I'm most happy about not having to go through that again for a few more years at least!