T i m wrote:

On mine it was fairly easy to
check and/or hardwire all the sensor inputs to both give some
supervised heating and also to see where the problem was.

I had thought of doing similar but without any idea what goes on in
the control unit (and therefore the in / output logic) I didn't want
to make anything worse.


If the control unit is very simple it will just switch open the gas
valves and it's usually easy to bypass the controller so that happens
if you're desperate for some heat and hot water. You just need the fan
and the flame (and the pump) and away you go. It's no fun hurrying to
fix a boiler in a cold house so I usually get the house nice and toasty
first before actually fixing stuff. ...although having said that some
people frown at an open boiler with a few wires hanging out of it :-)