Used these guys before buying my last car.
While they are not a full hpi check as in the uk, (insurance companies dont register cars which have claims against them as in uk), I found the service excellent.
I found cartell helpfull and for a change genuinely appreciative of my business.
I used there mechanic this time as I thought the deal I was getting was too good to be true, and felt better paying over the cash once he'd seen it as I had a qualified second opinion.
They are not the total solution, you still need a keen eye, know the questions to ask and do some investigating though.
Dunno about Data protection issues, all I was told was the number of owners, original colour, gearbox, registered written off, hackney, and importantly wether or not it had finance owed against it, I doubt this falls under DP Act. It told me nothing about the owner or his details, I had to do my own investigating for that.