Hi Howard,
After getting my car back on the road in 2021 (after 4 years off) the instrument and driving lights were very dim, in fact dangerous on unlit roads. The fusebox and lighting relays were my first point of call (I'll post some pictures separately). That gave me bright driving lights but did not fix the instruments. They were helped by cleaning the rheostat.
All did was to remove the collar fixing the switch (the right hand of the two chromed knobs) to the dashboard and squirt a little switch cleaner, from a pressurised can with a thin tube inserted into the nozzle, into the switch, around the edges of the shaft. I then rotated it a few times and did it all again. The improvement was significant and the instruments are now readable at night. I think the switch is still dirty, so I will remove it and clean it properly (or replace it) when I can.
I would not expect LED lights to be dimmable, so if you swap to LEDs you might be better off taking it out of the circuit entirely.
Glyn