What do you mean. Truth be told I don't have a burning £2000 hole in my pocket to fork out for a new boiler so I totally take what you say I'd rather run this thing till the bitter end. From what I've been told, the diverter valve needs new seals / refurbishment. After 17 years I'd imagine the heat exchanger, pump, etc are all running at the end of their useful lifespans and to replace all that might be new boiler money any way. We'll see and fingers crossed.
The receiver wasn't working because.....I did indeed have the switch positioned on the heat only setting. Very very stupid, I know....Problem solved.
New thermistor sensor is in, doddle to do. Oddly enough I got a decent shower out of it today at last prior to swapping the part. I'll run the shower again soon and see if I encounter E20 again.