Thanks for your replies they were helpful.
I have spoken to my local Volvo service department and they referred me to an auto transmission specialist, they advised me to get the vehicle into their workshop and hook it up to the equipment for a full diagnostic check over.
Approx £50 to £70.
They claim that the workshop equipment will check all of the electrical circuits and also “test drive” the transmission to see if it is responding correctly under different simulated driving conditions.
As for the flush, they both responded with a similar answer, a good idea as a preventative measure but they didn’t think that just changing the fluid would be the magic quick fix I was seeking, and if it didn’t work I had just poured money into a transmission that might already be dead.
I think that I have taken this diy approach as far as my limited experience and knowledge allows, so on Monday the brick is off to the tranny workshop for a very intimate hook up with a trendy tranny computer.
I will keep you updated, as I am sure that mine is not the only 850 with transmission problems. Thanks once again to those who took the time to respond.