I bought a old F650 a couple of years ago, which was fine.
About a year and a half ago i was looking at a 1150GS he had for about 4.5K, I noticed that the colour of the bike didn't match the reg docs and asked him about it, he said something to the effect "oh they must have got it wrong at the DVLA", did a HPI check and turns out the bike was a CAT 3 with the chassis of an adventure and a tank of a 1150. So he was trying to fob be off.
So examine the paperwork and do a HPI. Given the current econmic state, i reckon there should be loads of bikes out there for reasonable prices, so dont rush a purchase. (a mate of mine sold a '07 1200GS with 6K miles for just over £6K in July, thats £2.5K down in less than a year)