Well, what an event. Went and looked at it, checked it out, drove it everything seemed peachy, right down to knocking him down to £2k.
Got back to mine to complete the deal and do a HPI check, and it turns out it's a Category B, and on further inspection the vin plate in the windscreen has screwdriver marks all over it.
I feel for the seller, he was genuinely shocked and quite upset about it. I even asked him first if he minded waiting while I did a HPI check, and explained what it was (he'd never heard of one!), he was perfectly happy.
It is a shame.