Dead Shower HELP!!

I installed a new shower in August. Due to lack of pressure i fitted a salamander pump in the loft at the base of the hot water tank as per instructions. Pipe work then runs along the loft before dropping down into the cavity walls to join home of amber concealed mixer shower. This worked fine for a few months then became hit and miss as to wether any water would come out when the shower was turned on. It has now ceased completely. I have changed the pump thinking that was busted but the problem persists. When disconnected in the loft water flows as it should in the pipe work and pump kicks in, so problem seems to be with the mixer unit. Any ideas? Talked to manufacturer and they seem to think it could be the non return valves. Would both of these go at once?
Please help before i hack a hole in the newly tiled walls and the missus plays hell
Thanks

why wont the manufactuerer come out its still in warrantee?

I can't see much point in non return valves with a twin pump. Take them out. (But keep them just in case...)

I called the Manufacturers help line this morning and the have just said that it will be the non return valves. The problem is that to get at these i am now going to have to knock a hole in the tiles. I have my doubts as to wether these would both go at the same time and completely stop the flow of water. I thought it may have something to do with the thermostatic cartridge as that is the only other working part i can see. Are these prone to failure. Is the manufacturer obliged to come out and inspect it if it appears faulty? No doubt if i take it to bits as i was intending it will invalidate any warranty.
The non return valves are part of the body of the mixer unit, if i remove these wont it leak?
Thanks for a swift reply