Basically, i stumbled upon a way to determine if a car is a CAT D or CAT C without the need to buy a HPI check.
Go over to:
2. Click on 'SELL MY CAR' at the top
3. Type the REG, mileage, choose private seller. Then create ad
At this stage it should auto pick up the car details for you, if it doesn't you're out of luck with getting a quick free HPI check. However if all is well carry on to the next step by clicking NEXT.
4. On the right hand side you will see a column. Under 'Your Advert' it will tell you if it is a Insurance right off by displaying the letter D or C next to the asking price of the car. If you cannot see any 'C' or 'D' then its likely a clean car.
To test it out......
Go on to ebay or such like and find a CAT D or C car. Note down the reg on the pictures and put it into the autotrader website to see for yourself. Works only for most cars, not vans or anything else. Purely cars.
Autotrader get their info from experian, which is one of the top HPI companies which hold information about all vehicles. However, of course, it will NOT tell you if a car has finance or any other info you may need to know. I'd advise that if you're still nervous about buying a used vehicle, then do a proper FULL hpi check ie.. from hpi.co.uk or confused.com (who use hpi.co.uk system). Its expensive but also gives you information about any mileage discrepancies.
Hope it helps.