Maybe you can post some pictures.
I'll take some pics tomorrow and post them.
One thing is odd, you mentioned 4 pipes, I expect 5 on a combi.
Yes sorry it also has a gas pipe in the middle, and 4 water pipes, two to the left and two to the right The far right pipe (which the filter was in) is, according to the manual, "heating return", and the one to the left is the "cold water inlet". The filling hose connects the two.
The stopcock I mentioned is not part or the boiler, sometimes people put them in so they can isolate the boiler from the water mains.
Then I dont think I have one, just pipes going in from the mains and in/out to the radiators/taps. I dont see anything else connected anywhere along any of the pipe runs.
Do I understand it correctly that your system still uses the old pipes from the previous boiler? If they have not been cleaned properly, you could have debris ending up in the new boiler.
It also sounds like your filling loop is failing; that would be unusually early.
No nothing was re-used from the old system, the new boiler is not even in the same room. It was all put in from scratch.
What makes you suspect the loop is failing? Just to clarify when I said water was leaking out I meant after I had loosened the nut connecting it to the far right pipe. When I tightened it again it wasn't leaking, and as far as I can tell the isolation valves are OK.
Agile said:Just above and behind the pump is the auto air vent.
Is the air cap on the top just loose?
There is a screw sticking up which seems pretty loose - the cap that it is screwed in to didn't seem to move though.
I would still suspect a blocked plate heat exchanger.
To be honest a blockage of some kind seems to fit - since I had cold water until I drained and refilled it after which its been warm (albeit still fluctuating) it does make me wonder if the sudden flow of refilling it might have dislodged something?
I also noticed in the manual a "hall effect sensor" which apparently has a filter - should I try to get to this to check it too or is not relevant to the hot water system?