Glowworm Spacesaver Mark 2 gas boiler system prblem- not heating

My central heating and water not heating up . Gas boiler lights and fires up then shuts down before water and /or radiators get warm. Bled radiators and central heating pump. room thermostat appears to work -it clicks or raising and lowering adjustment.The grudfos water pump appears to work but could it still be faulty and not pumping through system hence boiler shuuting down? I have checked header and water supply tanks have water present in them. Any help would be most gratefully received
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