Thanks Ian. I had the PCB replaced a couple or three years ago. The system's absolutely dead - reset has no effect - so I guess you're right. Potterton say a solid red light means 'no demand' but I've got the thermostat cranked way up and controls on manual so there's definitely demand.
Can I replace the board myself? It's about £120 + VAT i think.
Mike Teddington
August 2009