Hot water pressure low in shower but WHY !

[macp]

So over the past two or three months we have noticed the hot water pressure from the shower dropping. We have an exposed Grohe mixer valve with the tap on the left. And the water temp on the right. As this unit had been in place for over 10yrs I decided to get another Grohe mixer and replace it. I did so today. And bugger me if its not made any difference. The really confusing thing is there is no pressure issues anywhere else in the house. And the taps for the bath directly below the shower do not seem to be suffering from low pressure. I should mention we have a combi boiler which was replaced about 3yrs ago.

Im bloody well stumped !