Confidence: It might be this.
Notes: Appeared in several pots, at first thought to be blackthorn but leaves much larger than the blackthorn round here. Then suspected crab apple but none seen locally, nor have I seen any local plum trees - only cherries and (planted?) appples. Sowed several apple seeds a few years ago. Looks very like "Type of cherry?" Observed by Ann Baughan which was identified by Tim Rich as P. padus or P. serotina. Growing on to see what flowers and fruit are like but could be a long wait!