Started investigating, and whatever the problem is, the thing works fine dissembled (tray slides in and out when eject button is pressed).
As soon as the first part of the casing goes on something seems to stop the eject cycle.
Took the fascia off (so it's not that).
You only need to disturb the casing less than a mm, and whatever is "catching" frees and out comes the tray.
The thing is I can't see anywhere for the thing to "catch", and there's no compressive force when assembled (so it's not "pinching" the sides).
It's just hit the bin, as I can boot off USB, but what a ballache.