Just got my August copy of Kitchen Garden magazine. On page 68, in an article called 'Anne's top ten feared crop diseases', is a pic of exactly how my plum leaves look. This is apparently 'blossom wilt'. What they say is: Shortly after flowering, apples, pears, plums and other fruit trees show dead stem tips often with withered, brown blossom and dead leaves at the ends. Prune off infected shoot tips and as one of the fungi responsible also causes brown rot, collect and dispose of rotten and mummified fruits.
So I'll be pruning off those tips tomorrow, burning them then sterilising the secateurs.