At least I found a burnt track, so its off to my local electronics engineer tomorrow for a full board health check, a repair of the burnt track and a test of the fan switch to see if that's the reason the blower motor has gone on strike too.
I'm definitely making progress, not only have I found a clear point of failure on the board but I've also got the impossibly stiff mechanical heater control moving nice and freely for the first time in my ownership