Glowworm Combi Boiler - Hot Water problem

[dcawkwell]

I have a Glowworm Combi which is very slow to allow hot water through.
Initially when the hot water is turned on there is a good flow of water but it's cold. It doesn't matter how long the tap is left running, the water remains cold. If I turn down the flow and leave it running very slowly for a while eventually the water will run hot. I can then turn the tap full on and I'll get a full flow of hot water. The problem only happens in the bathroom both the bath tap and the shower. The kitchen tap is fine. Does anyone have any ideas?

If the water to your basin/kitchen is fine.
You have a problem with the thermostatic bath tap on the bath. The
boiler is fine.

 

[John560]

I just spoke to my mate on the phone. The shower and the bath are in separate rooms. The symptoms in both rooms are identical so probably not the thermostatic water temperature controls. A good idea though.

I did have this problem on my shower a year or two back when the thermostat got scaled up. My instinct is that it's his diverter valve. Do you know of any good diagnostic that can be used to check it?