I have long had an urge to buy or build a small panel with a row of
LED's which shows the status of my CH system and wonder if anyone has
done something similar? Idea is to be able to quickly diagnose any
problems, so I can fix them. I have long since forgotten the wiring
plan type.

I have - a condensing boiler, stored hot water, radiators on a wireless
room stat, LCD time clock, pump and three port valve. All the wiring
and the wireless stat receiver, terminate in the airing cupboard, where
the tank and its stat are located.

I'm thinking - LED's for pump running, call for space heating, call for
water heating, valve position indication, time clock says heating on,
time clock says water on.