My brother altered some pipework to one of the radiators in my flat as he has done other times in other properties. The central heating was working intermittently afterwards but lots of air in the system. I have tried bleeding the system a few times now and also refilling via the drain cock. I am 99% sure that the pump is working and also the three way valve. The hot water isn't affected and is good. There seems to be some water gurgling back down the pipe near the hot water tank if I switch on the heating. I can hear the pump trying to force the water around and then when I switch the heating off again I can hear the water running back over. I think it is something simple that I am missing. Could someone lead me through step by step to putting this right please?