I know Britain's vast pool of TV drama character actors re-appear all the time as different characters in various series, but personally I take the view that if an actor is distinctive and idiosyncratic, they should not appear in episodes of the same series or related ones such as, here, Morse and Lewis as different characters.
In this Lewis episode, Lewis decides to try having an allotment to grow vegetables, and the man he sees about it is Mr Cooper played by David Ryall. There is no connection between the character and the plot. I happen to have watched this episode (january 2011) the same evening, when both were shown one after the other on TV, as the Morse episode Driven To SDistraction, where the same actor appears, and meets Lewis as well as Morse, but there he is playing a totally different character. Given that the two series are based in Oxford. and other characters like Morse's boss reappear but are not in every episode, we should be able to expect that a familiar face is the same person, at least unless they are heavily disguised by makeup and different performances. Ryall was clearly only made up as "himself" and not asked to give anything but his usual performance of an Englishman his age.
Both pof these are the usual interesting episodes, separated by years, and the colourful backgrounds with a city full of people are done well in both with this exception. It is just a shame if even years later a distinctive face recurs like this but as a totally different person.