Brute force with a small screwdriver and a big rubber mallet finally got the chrome bezel to move. With gentle persuasion I got it off.
There is no way into the mechanism without releasing the riveted brass connector block at the rear. If the rivets could be taken off, the whole mech could be withdrawn through the front. The dial is riveted in place too, so there's no way into the front.
I pulled the needle off easily enough, then spent a while putting it back on so it was a nice fit against the bottom (zero) stop. In order to get it right, I pressed it on with the needle on top of the stop - so it bent upwards slightly - and when the centre boss was in place, I flipped the needle to the zero point.
Next came a test ........
I inflated a spare wheel to an indicated 80psi - centre of the scale - then took the hose off. I then tried my digital pressure gauge at it - it read 72.8psi.
When I replace the track pump, it reads way low due to some internal valve system or other and I have to get the hose up to pressure and equalise pressure before the dial reads correctly(?) I bring it up to 80psi again, but the digital gauge is still reading 72 or so.
I wonder if the track pump lets some air out when it's taken off the tyre and therefore allows a lower working pressure.
I'm no better off!
Mick F. Cornwall