I was busy doing other bits and pecies, this plumbing job was left alone. However I got the parts for it this morning from a local shop and finally I'm on to it!
What I did was simply took off the broken mixture valve and installed a 100mm brass pipe instead. There was no need for the mixture valve to be there!!! Shower and the hot water tap in the sink in this particular unit have their own mixture systems.
So......What kind of SPECIALIST put it on to start with???
Anyway it's all fixed and this unit is now ready to sell.