Hi there. I have a Baxi Solo boiler connected to a Honeywell timer
control system. The control system consists of a ST699 time
controller, a T6360B Room Thermostat, a L641A Cylinder Thermostat and
a V4073A Motorised Mid-position valve. The entire system is installed
about 7-8 years I believe.

Right. The problem I have is that the boiler is firing up randomly
when it shouldn't be on. The timer says no, but the boiler fires up,
stays on sometimes a few seconds and sometimes an hour or two, then
shuts down again. Sometimes it will then come on again within seconds
or a few minutes. I've replaced the room thermostat and the time
controller to no avail. At one point the heating portion wouldn't work
at all but setting the lever on the mid-position valve to manual
enabled the heating when the hot water was due to be on. Now the lever
on that keeps shifting itself back towards the auto position, not sure
if this is anything to do with it.

This is becoming a real pain as the boiler is in a cupboard in the
bedroom and is very annoying if it comes on at 3 in the morning. It
generally is fine when the timer says it should be on, and I can get
it to work on demand, but it's the randomly coming on at other times
bit that is the issue.

Any thoughts on this? Please? I'll be your bestest buddy if someone
can help me with this. I don't want to call a heating engineer yet as
the expense with not knowing quite what is wrong is worrying, when I
mentioned this at the service a few weeks ago they didn't know what it
could be and I generally trust that company.