Well, we've got a chassis that works.
Now for the body . . .
Initial outline drawing photocopied, and glued to a lump of balsa, and roughly cut out:

Then some sanding:

. . . and more sanding,until it starts to look about right:

.
At this stage, I'm starting to wonder if the mechanism will fit inside it, so decided a quick resin cast would be in order:

Everything fits, but I made the mistake of trying out some colouring pigment. Looks nice, but what messy stuff! Think photocopier toner - only worse! I'm still getting a tiny spill off my hands - it gets down the sides of your nails, and quite honestly, I think you have to wait until it grows out.
The other problem is with a dense black shell, you can't see inside well enough to gauge how much more material you can remove.
So I did another one with straight resin:

and looked at it alongside Scalextric's offerings.
So far, so good.There followed a long spell of further sanding and shaping of the master, to reduce the overall size(these things are TINY), and get
rid of the woodgrain effect.At first I used coat after coat of cellulose-based sanding sealer(plenty of fresh air required - you can feel this stuff dissolving your brain), then filler primer, then automotive gloss, until:

This looked good enough to make another mould.
A big difficulty, I've found, is that when your eye is tuned to one scale(1/32), it's difficult to maintain a sense of proportion in another.
It helps to have a figure to put alongside the model:

The standing figure is from a 1/24 railway range, and gives a better idea of the sidecar's size.
If anything, it's still a bit bulky, but it gives the flavour of the original and I think I'll get it all working before refining it further.
. . . to be continued.
David.