Thank you Edgar, is there a publication I could find that shows this? The on from 41 Sqn above shows a two-bolt side fixing, but others have 4 bolts per side.

According to the leaflet, you need drawing 30030 sht 33, and it implies that you need to remove four already-fitted screws (2 x 6 BA & 2 x 4 BA,) then use the holes as a template to drill four holes into the mounting bracket 30030/541, countersink the 4 BA holes, then refit the bracket using 4 & 6 BA screws (not bolts) + nuts.

Supermarine didn’t start to fit mirrors until September 1940, but there are stories of pilots going to the local car dealers, and getting their own (your Triumph items, perhaps?)
In December 1940, a leaflet was issued, telling units how to fit their own, and it called for “26AJ/24855 Mirror, Desmo Type 44 (2 lug attachment.)”
An “improved” mirror was introduced in December 1942 (sorry, don’t know the type) on all Marks, except P.R.

First, take it into a darkened room, and wait for a few minutes; if you can see the interior tubes, or any other part, glowing, you are likely to have a problem with radioactive paint, and all that entails.
However, the dial, tubes, and cross-wires look so bright, it’s possible that it’s been “modernised” post-war, and the radioactive paint and powder (inside the tubes) has been replaced by fluorescent material. If so, you can relax (a little.)
The bowl should be filled with industrial alcohol (which is poisonous,) which stops the internal spider from rattling around uncontrollably. If the spider rocks around, and (worse) you can hear it rattling, the bowl needs to be refilled with liquid, since, if it completely dries out, the internal paint could flake off.
If the paint doesn’t glow, of its own accord, get hold of a u/v lamp, and shine it on the compass (again in a darkened room); if the paint, etc., reacts to the light, and glows, you have far less to worry about.
If you push the curved lever forward, you’ll find that the black bezel should be free to rotate, the idea being that you turn it until the heading you want is “on” the small tube level with the foot marked “fore.” (It’s under “30” in your photos.)
Then rotate the compass until the “inverted T” inside the bowl is lined up with “N,” and you are now flying your Spitfire (or whatever) along your chosen heading.

Mares eat oats
And Does eat oats
And little lambs eat ivy.
l

A kid’ll eat ivy, too, wouldn’t you

No, I wouldn’t.

The RAF Museum does have a lot of Bristol drawings, in its “MAC” range; they seem to be, mostly, for aircraft, but there are some with no designation. There are thousands of drawings, and they are not in a completely logical order, so prepare to spend some time in the library, going through them (60,000+, last time I looked.)

Hmmmm Are any WWII Spitfire pilots still about having sat for the best part of a number of years in these radiated cockpits?

And were they faced with instruments with damaged glasses, cases and seals, with 70-year-old deteriorating paint?

Funny, really, when you consider, only a few days ago, we were advised to get out more, since sunshine is the best way for us to absorb sufficient vitamin D.