Boiler overheat trips out (new ACB already fitted & pumps are running)

(owner - not a plumber so please excuse idiot-language)
Tearing my hair out: Glow Worm Ultimate 70ff & BM2000. BM2000 ACB failed. Replaced and now working properly. Sensors working too and all three pumps are running but boiler overheat trips out after an hour of operation. It also trips when the thermostat clicks off - which suggests the BM2000 isn't 'pulling' enough heat from the boiler... the over-run appears to be working for the 3 minutes Gledhill says is normal.

Other info: all radiators are bled - some indication of sediment in a few of them (cold lower middle). Boiler fires up when HW demand is called for.

The initial fault was, I thought, the ACB - but it could be that the boiler caused that to happen (I'm out of my depth on this). Two plumbers have so far been unable to diagnose. I'm in that Catch22 position: it could be the boiler, it could be the BM2000 - and the only option .. until I came across your forum .. was paying for fixed repair from both Glow Worm AND Gledhill (and I betcha one would come back with an unpractical-to-repair!).

I'm obviously not going to try any repair/replacements myself - all work will be done by a proper plumber.. I'm just after any suggestions to cut down diagnosis time.

Thanks

Nick

Need to check boiler pump.
And that the unit is full.
And it could be the unit, boiler and pipes from boiler to unit are sludged up.
Look through the BM 2000 posts.
Try a few things as above and ref the posts.
Then get back.
Mike