Thanks everyone. In the end I managed to give away (get rid of) most of them to colleagues, customers, neighbours and those that I had left over I just popped into the freezer whole (that's the plums, not the colleagues, customers or neighbours!). We'll see how they go (the same qualification as in the previous sentence).
I've got greengages and loads of other plums due in a couple of weeks, depending upon the dietary quirks and requirements of the local parrot population, so I may be in a similar situation again soon and I might try to make a batch of jam with the purple plums.