cold temp on thermostatic shower mixer tap??????

Hi I wonder if anyone can give me any advice.

I have recently had installed a brand new combi boiler, and a new shower/bath thermostatic mixer tap. Everything was ok for four weeks – but now my showers/baths don’t ever get hot enough (warm at best). The water does not get any hotter even if I push the heat on the mixer valve up to max. There is no difference if I reduce the flow of the shower/bath. It seems like ther is always too much cold water getting mixed in. Occasionally I will get a decent shower 1 in 10 but there seems to be no reason for this. The tap temperature gauge on the boiler is set to max and all of the other taps in the house are scalding when the hot is on, so it appears to not be the boiler. I am a bit peeved seeing as I spent so much money on it. Should I get a plumber out should I try and get a new warranty mixer tap as this one Is faulty?????
Mixer tap is metro vertical thermo shower bath valve from bathstore.com.
Any advice greatly appreciated
Thanks in advance

There seem to be two possibilities:

1. There's a flow restriction in the hot water supply to your tap;
or
2. The tap has developed a faulty thermostat.

Is there an isolating valve on the supplies to the tap? If so, you partially close the valve to create a flow restriction and see if the tap operates correctly. If it doesn't then it could still be either of the above causes, but if it doesn't then it points to a tap fault, in which case you should contact your supplier and ask them to replace the tap. Given that it's only been four weeks you can choose to claim under your statutory rights or under the terms of the warranty.