Have you got it working yet? If not;
I'm assuming water comes out of the shower from the
hot tap but the boiler isn't firing up - is this correct?

1. Have you cleaned the filter in the boiler?
2. If you run the shower and a hot tap at the same time does the boiler fires up and hot water comes out of the shower?
3. If 2 works then it sounds like the pump isn't producing enough pressure or there is a blockage in the boiler. Is there much difference in pressure (flow rate) between hot and cold water?
4. Try the shower tap 'upside down' - do you get hot water?