Help with cold thermostatic shower....

[Winestar]

When we run our thermostatic mixer shower when the central heating is on the temp is fine,nice and hot no problems but when the heating is off,as it has been the last couple of days ,the shower is cold,even turning the dial up to its hottest setting it's still barely tepid.
The hot water taps are fine,they take a while to get warm when the heating is off but are ok once getting warm whereas the shower just stays cold.
We have a Baxi combi 80e boiler I've tried turning the water heat up on the boiler but makes little difference...
Can anyone advise?!

 

[TCCHeating]

I suspect a part needs replacing in your boiler. If it is the part I'm thinking of. it shouldnt be too costly.

 

[picasso]

Worn diaphragm, the low flow on the shower isint activating the flow switch.(probably)

 

[Winestar]

When you say worn diaphragm is that on the mixer tap or boiler? Sorry i have no clue

 

[picasso]

boiler.