Even the free info on the total car check site leaves something to be desired - it shows the incorrect Road Tax band for my old 9-5 as L, when it is actually K. That makes a big difference! Also the estimated mileage is just a computer's guess based on the car's age, it shows 74,400 when last autumn it actually had over 125k on it. They don't offer a more accurate figure if you pay, but you can only get access to any mileage history recorded on VOSA's records if you have either an MOT certificate or a V5C anyway.
Their website also refers to stolen/recovered vehicle data from MIAFTR (Motor Insurance Anti Fraud and Theft Register) and the Police National Computer. While the first provides information to the industry, and possibly can be legitimately made public for a fee, I very much doubt that they have access to the PNC and reference to it on their website is possibly misleading.
Any website that doesn't have any contact details other than an email address doesn't get my custom. Also try Googling them and see what comes up on aol.co.uk.
