It didn't work
 

In case anyone is interested, I am here to report that without the o-rings there was a small oil leak once the engine was hot, so I had to persevere and fit the o-rings in the end. The threadlock was not the brand I usually use (it was honda threadlock 2, rather than loctite 243) and when I took things apart again, I could see that it had turned to a goo. (243 is oil proof and might have worked)
With the o-rings fitted, the oil leak seems to be cured - no sign of a leak after a 50 mile ride.
Mark