Thanks for the replies, everyone.
All very interesting stuff - at least I'll have somewhere to start if anyone asks me about this again in the future! Sounds like there isn't, and hasn't ever been, a specific route to becoming a F/E - it depends very much on the operator. But in general, it seems that some F/Es are very junior, hoping to get moved up, others are very experienced, possibly no longer being able to act as pilot due to the age 60 rule.
Cheers,
FFF
--------------