if the dash clocks have been replaced if faulty, then a vehicle can show a different mileage from what the dvla may have on file. if it is an honest repair then an honest seller would have proof they had changed the dash clocks and noted original mileage on the service book. pa dishonest can also purposely fit clocks with lesser mileage and sell on with ignorance. I would trust the MOT mileage over the quoted mileage. Another thing would be if the V5 was sent for change of ownership and the mileage was incorrectly written in when sent away could have caused the conflict of information. do they have receipts/proof this head gasket has been done. What are they asking for the car, people who travel distances to sell things are usually more keen on selling, won't drop the price to cover travel expenses and are further away from you encase of a come back. I would challenge the current ownership of the car, who's name is it on the V5? is it the same person that is selling it to you. if so get them to prove it. I wouldn't trust buying privately without making sure its the person selling the car actually owns it and not some fabled story of their cousin having a baby and getting rid. many people make a living just punting fixer-uppers, it's a fair game with honest people just making their money to get through, but you will always get the horrible sort always on the look to con you. one thing is when i person buys a fixer-upper from say an auction, they wont send away the v5 to transfer ownership onto themselves. reason being that it reduces the number of previous owners/easier to sell.