Hi Ivor, Pink grubs are a sign of plum moth. The only thing you can do is spray the tree at the right time. You can get pheremone traps which attract the male moth and when it starts catching them, it's time to spray. Here's a link to the RHS advice on this pest:
If you don't want to spray chemicals, the traps (about £6 to £7 each) used on their own may catch enough males to reduce the number of eggs produced by fertilised female moths to a level where you can harvest an acceptable proportion of unaffected plums.
DOES IT DO ANY HARM IF YOU HAVE EATEN A MOTH GRUB
WHAT ARE EFFECTS OF EATING A MOTH GRUB IN PLUMS
It won't do you any harm - I've done it and lived to tell the tale
Eating them is the main control method for raspberry beetles too.