Yep, Malcolm is right. If the seller sold you a vehicle knowing that there was a major fault that was not immediately obvious AND he did not tell you about it, he leaves himself open to being sued. Whenever I buy a car, or anything else secondhand for that matter, I always ask about major issues. If things subsequently turn out to be faulty, then the seller is liable. The term caveat emptor is quite limited and sellers cant just absolve themselves of responsibility. That's called fraud or hoodwinking. If you had bought the car knowing it had this fault, that would be your liability. Its all covered by the 'Law of Tort' in the UK & implied contracts.
Have a word with your local Citizens Advice Bureau about legal aspects & taking him to court if he wont pay for the repairs.
Contact a reputable Citroen dealer & ask them about how the fault manifests itself, given the mileage the car has done & general condition. Ask them about known issues with this selector mechanism & if there was ever any factory recalls on it.
Contact the seller, preferably by [b]recorded delivery letter[/b], stating the problem and that you had bought the car in good faith, that the onus was on him to tell you if there were major issues & that you will be seeking redress through the courts if he cannot pay for the repairs. Send him a quote from your local Citroen dealer. They don't need the car to price the work as its all listed on the repair & maintenance schedules for labour & parts.
Just remember, all is not lost. At the very least, get the RSole to take the car back & return your money, plus any expenses. If he wont, sue him.
H