A cat D car will not appear on the V5, only on HPI type checks.
Cat A cars are normally burnt out wrecks and nothing of value only scrap, cat B cars are breakers only and you need to be a registed breaker to buy these.
I've been told, by the trading standards people that a private seller does not have to tell you if it's a cat C/D car unless you ask, a dealer has to tell you if it is full stop.
I bought a car once not knowing it was a cat C until i got the log book back but he'd not told me it was, he said he did'nt have to tell me as i did not ask.
Fortunatly for me he'd wrote me a reciept on his business headed paper so i got some money back.
I went all the way to liverpool to see a 197 cup, 08 reg and it has had 5 owners, been on his forcourt nearly 5 months too, it's still on ebay.