Once the maggots are in the plums there is little you can do to get rid of them. Remove and dispose of the infected plums and don't compost! This will simply allow the maggots to continue to grow producing the moth that will lay its eggs on your plums next year. So into the wheelie bin with them. Do a good winter clean up on the tree.

Advice is to spray with permethrin, bifenthrin, pirimiphos-methyl or fenitrothian in early summer, with a second application 3 weeks later to control the young caterpillars as they hatch. (Don't know whether any of the chemicals mentioned has since been withdrawn from sale). You can get pheremone traps for plum moth which detect their presence and indicate the best times to spray, and if yours is a single tree, with no others nearby, the trap may disrupt the female's mating success sufficiently to reduce the number of maggoty fruits.