Well - it looks as though the problem is resolvable.
I bought a bottle of Polygen liquid latex, removed the bellows and clamped them open, then painted on the latex so as to fill in the holes in the mesh, inside on the bottom and them outside on the top. I allowed the latex to dry then turned the bellows over top to bottom so as to be able to get to the other side, waited until the latex was almost dry and replaced the ends so as to be able to try it out.
It is very much improved though I did miss a couple of holes, but I plan to repeat the procedure this weekend, allow the latex to dry overnight and then coat it with either French chalk or talcum powder, and the bellows should be airtight.
I rubbed off as much of the perished layer of old rubber as I could before I started, and used a No 10 artist's paintbrush.
For under £10 it seems an easy fix - the mesh is mechanically perfect, and is joined to the edges of the bellows, the box is hardly used, just left in its case since bought new. The problem is simply that the materials used do not stand the test of time.
I have been playing the box for a couple of days now, getting used to it after playing the D/G. It was a bit hesitant at first after its long lie in, but it is fairly roaring away now, and hopefully it will have a long and useful life.
If anyone would like to send me other examples of similar boxes then I would gladly repair and then add them to our teaching inventory - this coming Monday we are starting an after school club for music and dancing, there is the Maypole dancing and longsword starting up soon - all contributions gratefully received.