I wasn't there, but here's new information:
1.The boiler stat seems to be working. If I turn the dial (0 to 6 scale), it'll click at some point and the burner shut off.- The Immersion heater was not in use at the time
- The pump in the immersion cupboard is working fine on each of its three settings
- My mom told me today that because my sister was coming to visit and stay, she'd turned the boiler and wall stat up to their maximum settings. I would think the ch ought to have some safety cut-out for such eventualities, but I've instructed her in the correct use of the system. I've had it running all day without a problem. Nice hot rads, and plenty of hot water at a setting of just '2'. No need to turn it higher under any circumstances. I think that'll be ok for now. Mum is happilywarm again, and is going to take out a British Gas maintenance contract for =A312/month. So I don't have to do any work :-)
Thanks again Tony