Can anyone offer any advice - drained down hot water / heating system completely to isolate some rad pipes (had to drain system as nothing isolated). After filling back up (boiler - conventional not combi - myson marathon 600B) the hot water tank is not getting hot so no hot water - the pilot light is on on the boiler but obviously not transferring through to the header tank. Have advised may be an airlock as common on these systems but have tried to loosen nuts to header tank to get air out but still no joy. Any advice on this would be greatly appreciated.