Hydroflo VS2 - no hot water?

I have serviced as your clear instructions all the o rings in the above appliance. There was never a leak but I have a different problem.
The mixer wheel that determines the temperature seems not to be working. I have it turned completely up and there is only a luke warm temperature. The setting was fine before getting very hot water - but the reason I took the unit apart is because over the last few weeks the temperature of the water has been dropping until it was just cold water.
You can feel the temperature of the pipes outside the unit and the hot pipe is very hot. The barrel with the o ring and plunger with the 'hot cross bun' top has been greased with silicon grease. On the top of this plunger there is one 'corner' of the cross that looks like some entry for a jet maybe? When replacing this plunger should this area of the plunger be facing in any particular place or does it not matter?
Is there anyway I can dismantle the mixer unit itself as it does not seem to be 'mixing' hot and cold water! Is there jets or anything behind the circlip and spring.
I would appreciate any help on this matter.