Ok, so I have tried with a few different o-rings. If I do the one from the spares bag which is about 14mm OD, there is no way I can force the bell down over it:

The only o-ring (of the ones I have) which works is if I stretch the tiny translucent one (in the picture). Then the bell will fit over it, but sometimes the JFC gets stuck.

Is there not something funny going on here?