Just rounding off the thread by reporting a successful outcome.
I got a couple of sizes of Gorilla sleeve to try on my chain which is nominal 3 mm diameter link material (really cheap out of LiDL).
3/4" sleeving **LINK**
https://www.hypex.co.uk/Techflex-Sleeving-and-Heatshrink/Heavy-Duty-Sleeving/Gorilla-Sleeve/
fitted nicely. I imagine it would work with 3.5 and 4 mm chain too. Possibly 5 mm but that might spread the weave enough to make it hard to get on.
I slotted a piece of 3 core, flat, household mains cable through first to use as a stiff puller 'cos I had a bit lying around of suitable length. Tried a number of ways but best technique was to push on the sleeve to expand about 5 or 6 inches and walk it up the chain in short bites pulling it straight with the wire after each bite. Being flexible the chain won't stay put so the sleeve can just be slid on in the normal way.

Finished the ends by running a short length of adhesive insulation tape round the sleeve to pull the sleeve tight round the chain controlling the inevitable folding. Then two sizes of heat-shrink tube, the two largest ones from handy dandy set of assorted sizes from LiDL again, to make a proper job. Smaller size to neaten up the tape followed by larger size over sleeve and down onto the smaller heat-shrink. Just using the large size doesn't work out as it can't shrink enough to go neatly round the exposed chain link. Maybe self amalgamating tape would do better than ordinary self adhesive tape at the initial pull in stage as its much more stretch and flexible. Might be tricky to keep the chain taunt enough to use self amalgamating tape though.
Different bike in picture as it was the rotary Nortons turn for a run in the sun yesterday so I nipped out to B&Q at Bexhill to look at the sleeved chains they supply. Far too large for this job. Nice 70 mile afternoon amble in the October sunshine, almost too warm behind the Commanders fairing, remembering to keep a firm hand on the throttle to control the innate rotary conviction that 80 mph is a nice cruising speed. Everywhere!
Clive