My opinion, although im sure you will brand me a "hater" and ignore it, even more so as Im a mate of Matt's who bought it.
My honest opinion though is that as a car seller you have an obligation to warranty the car at least as far as being fit for purpose.
Legally I expect Matt should have brought it back to you (or got you to collect it if its not safe to drive potentially due to a misfire) in order for you to rectify the faults, but given the fact he is 400 miles away and they are only minor, I would have thought he has done you a favour by sorting them out and not expecting you to do anything other than make a 50 quid donation towards the cost of parts.
If I had sold the car, I would be fine with that TBH in the circumstances.