The only place to diagnose faults is in front of the boiler.
Unfortunately, you have not stated if this fault is ONLY on CH or on DHW as well. I will assume only on CH for the moment.
In this case when the boiler goes off it seems that the noisy diverter valve motor returns to the DHW position.
That implies to me that somehow the boiler is losing its CH demand.
If that's really the case then I would be wondering why that could happen and measuring voltages to see if external controls are functioning correctly with the room stat connections being the first place to check.
Tony
Your assumption is very correct @Agile the DHW is working fine it is only the CH that is playing up.
I bought the house (I have lived in for 20 years) off my mum a few month back, she says she is pretty sure the thermostat got disconnected when the boiler got installed. She may very well be wrong as its still on the wall, so I will see if someone has been messing with it when i finish work