No Hot Water At All?

Hi, I am having problems with my hot water - both central heating and hot water from the taps. The water from the hot water taps and the central heating is cold, it will not heat up at all despite the gas boiler working. I have replaced the 3-way Honeywell mid-position valve (head and wet end). I have also checked the Grudfos recirc pump, which is working fine.

If I leave the heating system off for 4 or 5 hours and then switch on again, the boiler fires up but switches off after about 5-minutes. It is like the gas boiler is sensing the temperature is too high, then shutting down the boiler. Although the recirc pump and mid position valve are working fine, it seems like the water is just not circulating from the gas boiler down stairs, through the recirc pump and mid position valve up stairs, to the boiler water storage tank etc. I don't know if the system is air locked, but I cannot get hot water from the gas boiler to the mid position valve area. I have tried everything - no joy.