try and go for a decently long test drive .Any honest seller wont fret if you ask that ,but a shark will ! at least a half hours drive ,give it time to get properly hot . i would ask the seller who has been doing the maintenance on it lately and then ring the garage up,they should tell you if its a gem, or not .
you can also easily look up on the governments m.o.t. website and this will tell you what m.o.t problems it has/has not had each and every past year . also this website checks on the mileage reading ,at each and every past m.o.t. , this proves if it has ever been clocked