Thankyou all for the wise or witty responses...
I removed the dimmer, pried open a corner and squirted in some WD contact cleaner - not sure if that did any good, but hoped it wouldn't hurt. Then cleaned the contacts / spade connectors with WD and tested it with a nail between the contacts first to see what the brightness should be without the dimmer involved. Then reconnected the dimmer and amazingly I can now see the numbers on the dials in darkness again, obviously only to Elizabethan standards but a tremendous improvement. At circa £50 for a new rheostat the £5 spent on contact cleaner was a bargain.
