Covering Neil Young - tricky territory because I'm guessing there's always the temptation to sound like ol' Neil when you sing. Mark's guitar was handy ("Hi Mark!"), but I think he could work on making the vocals more his own if he wants to loosen more passers-by purse-strings. (Just an opinion)
The busker who may rank as the best I've ever encountered to follow this principle was a chap doing a totally Smiths set - and his angle was to be completely cheerful in his interpretations. It just so worked - and he drew an eager crowd.