Valliant boiler with low water pressure?

Im having exactly the same probrem with my Baxi Eco 80 (which apparently is mechanically very similar). 3 months ago i had a new micro switch fitted because the rad kept coming in instead of just the hot water.. Last month, i had a new diverter valve fitted 'cause the water kept going hot/cold.. It was great for a couple of weeks... but this week, the running hot water pressure has dropped to a fraction of its previous pressure.

Everyt 3-4 month I have to open the water inlet valve to keep the pressure up. Now this seems to have no effect and i cant even hear the water gushing when I turn this on...

Im in a block of flats and the cold water runs great - so i assume the the cold water from the mains to boiler is also running at the same pressure. Is this a fair assumption?

Any ideas? Is this likely to be problem with the water supply or the boiler?

Peter
March 2009