If you use a machine with a really hard platen on a regular basis, you will eventually flatten the typeface (type slugs).  Once this has happened, the machine is ruined.  There are no replacement type being manufactured so other than labouriously transferring the typeface from an identical machine - or swopping all the typebars over and then making adjustments - you have pretty well had it.  The Continental Standard is the 'Rolls Royce' of pre-war manual typewriters and really worth looking after.  Backing sheets will help a lot, but the real solution is a re-rubbered platen.  It will make the machine quieter and more pleasant to use too.

 Scalextric tyre oil?
A few questions from a new typewriter ally!
Wintergreen oil for hard rubber 
On the subject of platen rubber 
punching holes when typing
How hard is too hard for a platen? 
Replacng a platen 
Backing sheets 
Different platen types/hardness
Different Backings for Hard Platens 
Backing sheet
Silicone Spray to Rejuvenate Platen/Roller Rubber? 
Repairing/resurfacing rubber
 

thetypewriterman wrote:

Here is a link to a firm in Holland that should be able to help you with re-rubbering https://www.akblongs.nl/contents/en-uk/about.html

Unfortunately they do not replate platens anymore. They used to, though. But not anymore.

Anyway, will give the heat shrink tubes a go; if I fail on that, I can always send it off for professional repair and hope J.J. Short won't laugh at me for trying heat shrink tubes haha.

As was previously mentioned, a Continental is more deserving than heat-shrink tubing.