Not sure what to do, I'm not sure it's worth the bother of pheromone traps/spraying for one sparsely producing tree. The fruit is good, but not as outstandling a yellow plum as the catalogues suggests, and the few unaffected fruits ripen very unevenly, the area near the stalk shrivels whilst the other end is sometimes hard.
My husband's favourite plum is the Warwickshire Drooper that grow rather ferally in his parent's garden, as unusually this variety does very well on it's own roots. The flavour isn't as rich as Coe's, but large, sweet and juicy, skins are equal in terms of thickness/bitterness, but it very reliable, seemingly pretty immune to silverleaf and only mildly affected by moth. The drooping habit is attractive, and somewhat self-limiting re. height which is nice.
I know the poor soil here means that plums are very slow to come into production, maybe it's simply too poor for a fussy variety like Coe's. I'll give it one more chance.