Thanks to advice from our very own Robert Smart it turns out the two studs that hold the top of the instrument pod actually terminate in balls and a hearty tug will release them from their ballcups and thus no need to fight two small and inaccessible nuts...
I had the opportunity a couple of days later to test that on another XM with a dead gear indicator bulb. Being someone else’s car I tried a gentle tug on the top of the pod but it wouldn't move easily so I removed the two nuts.
I could then see the studs were ball-ended and sat in clips. I replaced the nuts and tugged the studs with a pair of pliers and out they came and because the back of my hand was so near the top of the dash cutout I also lost a load of skin too

Putting the pod back in was a breeze. Replace the studs and nuts before refitting and click and it's done

The owner of the XM is 50p richer now. 50p and a big needle he uses to adjust washer jets was retrieved from behind his instruments.
I'd wondered what I could hear sliding around my dash for some time and when I did my bulb I found a coin there too but not so valuable as that. It was an European 10 cent coin.
I also advised how to do the starter relay mod on another V6 XM today, the one belonging to Chris570. His was getting harder and harder to start and sometimes could take up to 20 turns of the key before the starter motor would play... He stood this for months. How I don't know, mine only did it occasionally but even then it bugged me to the point I just had to do the mod.
A V6 Xantia also visited with an intermittent starting problem but that was found to be caused by the gear position switch on the gearbox that permits the starter to operate in P and N only. They do fail occasionally.
Before fitting a spare I had a go at fixing the old one. they're riveted together and easy to disassemble but that's as far as it goes. Inside they have a set of naked microswitches and the innards are made of cheese; any attempt to try to disassemble further for contact cleaning breaks a bit of critical cheese off and ruins the switch. So, if they fail then replacement is needed. Only problem is they cost getting on for £110
