Problem sorted. The old sight glass pushes straight out with a small tap from a large hammer. Behind it is a rubber O-ring sitting in a recessed groove. I found a broken pair of binoculars and took out the ete piece. Put that in the hole and gently tapped the metal retaining ring back in to hold it in place. Seems to be sealed OK.
Now if I can just fix the trottle retainer on my ISO grips, I'll be in with a chance of getting to Dent.
