Thanks to everyone for the guidance.
I found a source for the Dowty/ Bonded Seal washers, and have renewed the O-ring.
Could not get the V bolt to align with 1 or 2 Dowty washers, and 3 was too many. So have resorted to mastic like others to complete the seal. Not very happy with that solution, but it seems to be accepted method so we’ll see how it goes.
Now part way through replacing the bushing and lip seals, so will be able to fill the transmission with oil before wintering. A little tricky finding ‘silicon’ grease around here on a Sunday :-)
One final question on this little project. I remember having previously seen discussion about which oil to use. I have both 15W40 engine oil and 80W90 gear oil.
Which should I use?