Firstly, thanks to A. Lee who said
"I used to recommend Honeywell, but I too have had some failures, the 'down' button on my own has stopped working.
Anyway, Plumbcenter have an own-branded wireless programmable stat for £51:
It is a re-branded Honeywell, albeit an older version. I've fitted at least 6 of these with no problems yet."
I bought one of these, and then thought "Re-branded Honeywell? I wonder if it will talk to the controller?"
So I followed the instructions and although the thermostat unit didn't display the messages I was expecting it did manage to bind to the control unit.
So subject to testing I have a working system again where I can actually read the LCD display.
Although not quite as cheap as the £40 JoeJoe paid for a replacement controller, it looks as though the PlumCenter thermostat unit at £51 could supply a replacement receiver/relay box compatible with a CM927 over the counter.
Cheers
Dave R