I thought I'd update this as I have now sorted the problem.
Today I got under the car and the first thing I noticed was the back half of the solenoid was lose. It is clamped on with a strange bracket and it was lose. The solenoid was also lose in the housing and so i removed it all and also the bottom plate and filter.
The filter was clean and I also tested the solenoid and cleaned it out with mystery oil. I reassembled everything correctly and tightly and took it for a long drive. It performs perfectly now and no longer engaged on its own.
It was definitely the solenoid causing my issues and reassembling properly resolved the issue.
Very happy days indeed
