hello all, novice looking for some pointers from a kind soul. heating has been sporadic on and off the last couple of weeks, seems ok most of the day and then when called at night for an extra hour, from time to time I get: 1) banging from the pipework around the boiler 2) central heating not working 3) pump very hot but only luke warm attached pipework 4) noticed today some water leaking from the valve in the pic below the radiators have all been bled - there was no air in them. I switched the system off and unscrewed the central pump nut to bleed it - lots of air escape, no water at all, and the pump seems to be rotating freely. Checked to see if the valve was rotating and engaging the microswitch to trigger pump and boiler when calling for heat - havent yet got a multimeter out in the connection box to confirm, but it seems to be working in order, free movement. im assuming that all means an air lock - any help on how to resolve would be much appreciated!