Thanks Colin!
Ok - here are four pics.
Pic 1 shows the top plate (as I mentioned - a different shape to the Canley diagram - and it IS an overdrive)
Pic 2 shows the new cup and circlip
Pic 3 is a pic of the end of the remote (single rail) showing the existing plastic cup embedded in the metal seat.
Pic 4 shows a shot underneath the plastic cup.
Now, (if you're still reading...!!) I can't get enough access to the plastic cup as it won't slide back any further. I assumed I would have to remove the circlip from the bottom of the existing cup to lever it out, but looking at it, it seems the circlip is not visible, so does this mean I literally use brute force to lever the old one out, fit the circlip BEFORE I put the new cup on, and tap the whole lot home?
Or am I just mad.