After having a while of not being able to do much modelling, including settling into a new flat, new job, a fair bit of travel, etc. and getting hacked off with the lat couple of bits of the engines, I've finally got some progress worth sharing !!
These were the two rings that caused so much frustration. They each sit on the ends of 6 'ribs' and none of them seemed to be in the right position. I managed to choose 2 ribs that were more or less the right distance apart and glued the rings to them. Then the remaining 4 were encouraged into position and glued one by one.

I finally finished them last night, and now have two engines ready to fit.

Fitting them isn't likely to be easy. There are no locating parts to tell you how far forward they should go. There are notches in the engine housings that get them in the right axial position, but they are free to slide backwards and forwards.....