Suprima 30 & Boilermate 2 - Constant running and now lockout

Last few days the boiler seems to have taken on a life of it's own. Boiler has been firing constantly for a few mins then shutting down. It didn't appear to be getting temp info to say it had satisfied demand. Both water and rads were running nice and hot. My first thought was that the heat store thermostat may be the culprit as the green neon light at the base of Boilermate wouldn't go out regardless of where the summer/winter temp control was set. Whilst all was obviously not well at least we had heating & hot water so I felt it was not an immediate issue. However, boiler has now red light locked out - the green neon has at last gone out so perhaps I can assume that the thermostat is ok? or would it do so because of lock out? The small header tank on top of the Boilermate is full of very hot water so much so that it is transferring heat a good way down the cold feed pipe and running out a bit down the overflow to the outside world. The ball cock float in there appears to be working ok and draws in cold water as I believe it should although not necessary at present as I said it contains plenty of water, albeit hot. So from the symtoms do you think I might need a new boiler PCB? (had a new one about 4 years ago) or just a store thermostat? Is the latter quite easy to replace? I can cope with basic DIY as I have replaced the boiler pcb and both the pump pcb's in the past. Any advise would be most welcome.

You could have a few issues.
1; Cold feed on BM II gets blocked.
Unit and pipes from to boiler may be sludged up.
The unit may be under sized.
So when you are running heating and DHW the unit will need heating up by your boiler.
The BM II needs to be 70 plus c. So this time of year with the demand boiler will be running.
To find out if satisfied turn the heating off. Let the unit heat up to max.
Could be the pump on the BM II (boiler) the other is heating pump.
Header tanks will get hot.
If to much water take some out. Check ball valve is NOT dripping.
If ok make sure level is low in the header tank.
If after that it rises up and overflows would point to a sludge problem.
Also if you read all the posts on here ref BM II they will help you more.
Test the store stat.
Mike
MGC/H2O
Norwich Norfolk

Mike, thanks for taking the time to respond. I'll be having a look at what you have suggested and will post again if and when sorted in case it helps others with similar issues.

Hi Mike, on checking thru your list I noticed that the BM2 pump was very quiet and on further investigation it was actually dead - no power reaching it. Luckily I had a new pump delay timer pcb tucked away having got one when it had previously started the standard buzzing. However, as the buzzing had subsequently disappeared I had assumed it didn't quite yet need replacement - obviously it did and with the new one in, hey presto all is well in the Boilermate world again. Hot water and heating a plenty and a boiler that is now responding properly. Thanks again for your pointers, you are a diamond.

Thank you.
The basics! And logic.
Well done
Mike
MGC/H2O
Norfolk