Plums (and damsons) produce flower (and hopefully fruit after!) on mature wood, i.e. as you said, 2nd year wood and older. The most flower will be produced on the healthiest wood, which tends to be the younger stuff, but damsons are famous for fruiting prolifically as old trees, so there's a clue as to how they function

Thanks Worthy. I don't know whether its poor soil or what, ( I did give it a seaweed mulch in the autumn)but the new growth is rather spindly and because of an attack of something last year there are quite long bits of stem with no leaves. I trimmed it quite a lot in late spring, but as there is no fruit because of the weather I was wondering whether to be a bit more drastic.