The shower in my ensuite is the highest flowing water in the house.
Its on the regular hot water system (ie not a powered electric shower)
It has a Mira control for hot and cold water.

Hot water pressure in the shower has always been "weak" and hot water was always slow to start flowing. Saturday morning hot water stopped altogether. Cold works fine in the shower but if I push the lever to full hot - flow drops considerably and just some cold water comes out.

There is plenty of hot water everywhere else - including the sink and bidet beside the shower.

Here is what happened.
I turned on the shower - at hot. Normally, to speed things up I turn on the hot tap in the sink. (I do this because I was told that this gets cooled water out of the hot pipes and lets recently heated water through more quickly)
Anyway this time the result is that the flow in the shower slowed - but the hot water never came through.

Is this an airlock somewhere - or a blockage in the mixing shower control.

Any suggestions?