I got the end cap back on by pushing quite firmly - surprisingly firmly in fact. Slightly complicated matters slightly by not having the 3 holes on the gear side perfectly aligned with the holes on the mounting doodah, so had to push the cap out again, realign and reassemble. A 5 second test flight showed rotors running smoothly so looks like the 'repair' went OK.
However on next flight I very soon smelled something burning so immmediately shut down and started sniffing my new toy, extremely paranoid about screwing up this thing beyone repair. But as you know, when you start sniffing around for a burnt odour, you sooon become de-sensitized and ca't smell a damn thing anymore. Hmmpppph - still don't know if I'm burning my toy. (I don't think so though - probably a neighbour burning something)
If anyone has the same problem with the motor end cap, a couple of comments:
The 3 short screws are on the rotor side, longer on the opposite
When you push back the 'cap' of the motor into the motor housing check the position of the screw holes on the cap - they should be aligned with the housing where you're going to mount the complete assembly. Very difficult to turn the motor once its inside the mounting frame.
I should have put some loctite on the cap before pushing it back in, but was unsure if it would really go back in. Once in, I was reluctant to get it back out and start from scratch. But it is quite snug - can't figure out why it popped out in the first place.
I would imagine that if I place ANY orders for spares I'm going to ask for a bunch of those tiny screws as well as the tiny circlip that holds the rotor on - I was more lucky than skillful in that I didn't lose anything. Will also order a few of the big gears attaching the rotor, one of them is already with a slightly 'rough' spot.