As I still had the very first PCB the boiler came with (changed due to usual intermittent lock-out issue) I put that in and hey-presto, it worked!
However, when I first turned the mains on I was getting a flashing red. Pressed rest and away it went.
Got another board from CET this morning, put it in and same flashing red light on power-up and I can't get the board to reset. Put the very original (very first) board back in, powered on to flashing red again but it reset and away went the boiler.
I'm trying to reset with CH & HW off at the programmer and with the boiler stat off.
The fault finding flow chart says things like check pump wiring isn't reversed but it's worked fine for years and indeed is running at the moment.
Anyone have any ideas, please?