Y-plan system sometimes stuck on running

We've recently moved to our first home and it features an officially "non starter" boiler.
When viewing, I thought I recognise the problem.
New PCB and the system is running!

Now I am trying to troubleshoot the next problem:
The system works for most of the day alright. Heating and Hot water are available.
However, around the time I set it to make hot water, I notice it then doesn't stop, making the house a sauna!

I thought it's the thermostat and we had a hand-me-down hive that is known to work. I installed that but it doesn't seem to fix the problem.

I have a workaround: When I shut down power to the heating system, I can hear the Mid Position Actuator (Drayton MA1) revert to some position and then I turn it on - the system would work fine again, until the next day.

I tried some guides online but got confused and not sure what it should do and what it shouldn't.

Is there a step by step troubleshooting guide for this that you know is pretty straightforward?
I got a multi-meter as well.

I realise this isn't the first time someone ever asked about this and I did try a lot of the online resources but I am stuck clueless.

what do you mean by hand me down hive ? is it connected to the internet ?

It's second hand from a friend. Pulled out of a fully working similar system. I mentioned that in case it might shed light on what's going on
It is connected to wifi but I didn't set it up to interface with the www.
I think I'll switch back to the simple thermostat, just to diagnose a little easier.

Is there like a "most common" diagnostics routine that should be tried first?

I assume (and I hope!!!) that the actual valve is OK, just the Actuator sensor is stuck.

you can not move a hive from one system to another , unless you know the user name and password from the original installation, if you dont you will need a new Hub everything else will be fine you will just need to link the new hub in and start a new account

this is not about the hive controller, which I was able to install just fine
The heating system has the same problem with the old thermostat and hive. I just mentioned that to rule out the thermostat.

Does your Y plan have a connection from the DHW off terminal in the controller? If you are picking up the live feed to the auxiliary switch from your switched feed to the y plan valve, it will do exactly what it is doing. However, the MA1 actuator is notorious for developing this fault after a year or two.