dear all, quick update - after contacting a plumber and waiting for a response, I thought I'd make some more checks: checked all the connections in the connection box with a voltmeter, and it seemed that everything was being triggered and live when it ought to be. the pump needed bleeding again, and twice I had to encourage the impellor to start rotating with a little screwdriver nudge when there was air in it. not sure whether this means its on its way out, or whether thats normal when theres an air pocket in the system? after the suggestion of looking into automatic air release valves, I ended up taking apart the two that are in that first picture, which was easy to do by rotating the body to seal them off first. One was leaking quite a lot of water, but I had noticed that both were releasing a bit of air. both had fine, silty gunge in them, but seemed in good knick, and were very easy clean and put back together. the result is that everything seems to be running perfectly! no worrying kettling, no scary bangs, the pump seems to be running fine, teh valves seem to be operating when called, and the heating comes on when expected now. will keep watching to see whether this is just a temporary reprieve, but thanks very, very much to everyone who took the time to reply. @Allsorts 's post in particular I found most useful! so that results in the following queries: 1) can a leaking AAV cause air to be drawn into the system? I'm wondering if the flow of water creates negative pressure that then draws air into the system (kind of indicated in the schematic below) - since cleaning the AAV's seems to have resolved the air lock issue. 2) if there is an air pocket, is it possible for the pump to become seized? thanks again all!