Well, thats to Tony (TTC) above, I have sources an olive that seems to have done the trick, so I should be back on the road. I think the long game with this will be to decommission the oiler system, although the coupling which has caused me grief is the one running from the block towards the pressure gauge, so this will remain, won't it?
You are not the first this has happened to. You could do what every manufacturer did when the
DVLA put their foot down. Install the electrical version of your oil pressure gauge. Looks identical.
The sender can thread at same outlet the tube comes from. Then you run a wire from the sender to
the new gauge. No oil lines.
Alec