Having dismantled the dashboard, I had a look at the instruments from the instrument panel, as it was my intention to replace the ordinary bulbs with LED lighting. I'm afraid the photos are a bit blurry except for the last one, as obviously I did not have the close-up setting correct!
Now why did I decide to do this? Well, there is a green plastic "shade" inside each instrument body housing, with a screw thread which attaches to a black plastic tube, into which the bulb fits. These had been broken off in the past (possibly due to poor design?) and had been repaired with silicone sealant. This photo shows the bulb in its holder with the green plastic threads of the "shade" still attached.
The broken-off plastic shade:
With a pair of long-nosed pliers and some side cutters I managed to remove this broken piece, leaving a clean hole:
Using some 15mm diameter copper tubing, I cut some sections of this to form a new tube to hold the light bulb, fixing it in place with epoxy. The internal diameter of the tube needed to be 12.5mm in order to hold the bulb firmly, so I cut some slots in the copper pipe in order to reduce the diameter a bit. It now holds the bulb quite nicely:
I will be installing some high-powered blue LED 501 bulbs. These are a direct replacement for the ordinary instrument bulbs; no modifications are required.