You need to take the speedo out of the dash Rob. IIRC correctly it’s two little spring clips inside the bonnet side of the dash behind the fuel tank, that you have to pinch together to remove it from the dash. You will obviously need to undo the speedo cable and pull all the warning lamps out.
Once you have the speedo out on the reverse side you will see, I think it is 3 or 4 little tabs bent over that are holding the bezel in place.
I did this job about a year ago as my speedo "glass" was badly stained/sunbleached. Tony is quite right - it involved removing the fuel tank, the speedo cable and, ideally, the various warning light connections. It was easier to remove the whole speedo binnacle (a couple of squeeze clips I seem to remember) to get better access to the small (and very delicate) metal fold over clips holding the speedo rim (there is also a rubber foam around the inside edge of the speedo).
It actually wasn't that difficult (and I am not very experienced). The biggest headache was discovering that several of the electrics didn't work properly after I put it all back together :bang: ....eventually traced (thanks to a knight in shining armour who read this forum ) to a dislodged/poor earth connection.