New rubber grommets. Outer diameter a bit smaller than the original and inner diameter a bit bigger, but a decent fit. To compensate for the thin plate of the frame, some packing string soaked in plasti-dip was wound around to act as extra 'gasket' or filler.
Ergo a second attempt; now with heat-shrink tubing. This time only the two screws of the right-hand platen bracket from underneath the carriage base and one that holds the margin bar are removed to wrestle out the platen (black arrows).
With these three out (taking care to catch the nut of the inner bracket-screw) and the margin-bar nudged to the side, the platen can be wrestled out to the right-hand side. (This works easier when also the ruler-bar is removed, by the way. The ruler comes off very easily with loosening its two screws at the outer ends.)
The platen out again, the ratchet and linefeed-release need to be unscrewed. This allowed the inner-tube to slip off easily (talcum powder!) and the new heat-shrink tube to slide on. This one has a diameter of ~41mm and can shrink to ~19mm. Shrinking it to size really needs a heat-gun; a hairdryer works but really doesn't get hot enough. Making it shrink evenly without creases or bubbles probably would go better with more practice, but overall it shrunk on fairly well.
Re-assembling the machine again, with a shiny new platen surface! The nut that holds the inner of the right-hand bracket screws is a bit fiddly to fit - a length of sticky tape on a small cardboard strip helps to keep this in place when re-assembling.
Linefeed and indexing are all haywire now. Beginning to have a suspicion of the causes. One was incorrect mounting of the ruler (easy fix), the other could be to do with the ratchet (all needs to come apart again...).
Getting to know the machine :-)