I did some more on this over the last couple of days... The first job was to turn up an arbour on the lathe, which will hold everything together.
The plan was to mount the arbour in the rotary table, add the ring gear, then fasten the pulley to the remaining shaft that was sticking up. But then, the ring gear would a) be underneath the flywheel, and b) not tight, as well as c) blocking the t-slots in the rotab... After much

, I figured this: Mount the arbour on the mill; centre it (which, as it happens, also centred the rotary table, bonus!), then drill a 1/2" hole in the centre of the arbour.
Meanwhile, on the lathe, having located a suitably sized piece of scrap steel, I turned up a section the right diameter to pop into the rotary table, and then cut the end of that to fit in the hole I'd drilled in the end of the arbour. When the two bits fit, I cleaned the shoulder up, and glued them together with some of that fancy loctite:

So, now, one assembles the contraption thus:
Step 1: Arbour in rotary table hole, add ring gear:

Step 2: Add the pully, roughly centered:

Step 3: Fit the taper bush, using a bit of gentle tapping where needed. Then screw firmly into place using the supplied set screws:

Step 4: Finally, turn the whole lot over & drop the other end into the rotary table centre. Add a fairly precarious looking pair of clamps, and one is ready to commence drilling:

The procedure for the flywheel side is simple: Spot drill the first hole; rotate to 120
o, drill, rotate to 240
o, drill. Then open each hole out to 5mm, 8mm, 10mm in turn, now it's a clearance for an M10 bolt. There's a bit of a gap between the ring gear & the pulley, so I've been able to open the holes in the ring gear to full size without moving anything.
Now, remove the ring gear, and drill the pully to ~3cm depth, at the same 120
o spacing. Drill them out to 8.5, then use the tapping head to tap each one for M10, as deep as the tap would go. No snappage! (phew!). The pic is after spot-drilling the three holes, prior to drilling the first full-depth hole:

And here's the finished article:


The next job is to cut the keyway into the ring gear. I could do this just by drilling a chunk out (after all, it's hardly going to need the keyway itself for any reason) - but, I recently acquired a bunch of broaches - including one of the correct size - so I figure what the heck, let's cut it:

The only bummer is, I don't have a 32mm guide, to match the shaft size.... So, back to the lathe to turn one up.... Here it is on the mill, about to have it's slot cut:

....and that's as far as I got.
More tomorrow, I hope!