For reasons unknown it seems impossible to source a new filter - as various shops are discovering.
Rather than fart around, I've taken a look at the beast, and believe I can probably make do & mend.
The filter comes as a cartridge which is made of plastic. There is a retaining hexagonally-punched circular plastic ring which is spot-glued/ welded to the body.
My current thinking is to use a knife to sever the welds, and remove the plastic retainer. At that point, I can remove the old carbon (which seems to be in a mesh or net) and replace with fresh activated charcoal (available for 2Kg/£11).
The only issue then is to be able to replace the plastic retainer securely. Ideally in a fashion so that I can repeat the process ad infinitum (in the anticipation that the new filters are going to be impossible to source).
This leads me to a couple of ideas.
1) a series of screws from the outside in (there is plenty of room where the filter is located, so no worries there)
2) some form of internal expanding circlip which will hold the plastic down firmly. However, there is no groove inside the housing. I am imagining some sort of inside-out worm hoseclip that can be opened with a spanner to grip the sides.
as a final complication, whatever the solution is, it needs to be water (and alcohol :) ) resistant. So steels and irons are out. Brass or copper ideally.