Thanks to previous posts and links for helping me quickly understand the physical set up.
Main thing to realise is that there is a metal plate behind the glass that makes it very hard to clean.
Some of suggestions here, like drilling and breaking the glass, or total replacement seem a bit drastic for a few microns of carbon deposit.
To get easy access you need to remove the oil filter (which you are probably doing anyway) and the oil scavenge pipe ( one hex-screw ).
Physical access to wipe the inner surface is near impossible so I tried squirting some WD40 cleaner/lube spray into the holes. It probably needs acetone or similar but I don't have that in spray format. Electrical contact clearer may help.
However, I did find that I could push the plastic straw that comes with the WD spray through the holes and get it do a loop inside. Then pumping it back and forth gets it to rub the glass. Try different holes for different directions.
A final blast of WD40 to flush it out.
Something like an old-fashioned pipe cleaner or the micro brushes for getting between teeth may do better.
This solution was sub-optimal but does clean the glass enough for me to see the oil level once again, which was after all the aim.
No breakage, no chasing spare parts, no cost. Oil change completed, back on the road. Near enough for my needs.