Strange thermostatic shower

[blueagle78]

When I got my house refurbished, I got a new thermostatic shower installed. It worked perfectly until recently. For last couple of months, in the morning I'll turn it on and it'll take anything between 2 minutes to 30 minutes before water would come out with a normal pressure. We leave the hot/cold controller to one set place. But if I turn the hot/cold controller knob to cold then water would come out straighaway and if I turn the knob back towards hot side the water would stop.
Could someone tell me what could it be?

 

[seco services]

is it running on a pump ?

 

[blueagle78]

I'm not sure what you mean, sorry
I've got hot water tank in the loft and I get hot water in the sink and bath tabs within seconds (as soon as cold water has been run out of the pipes). Even when sink or bath tabs would be running hot water, shower would take quite sometime.