The DNO gentleman has just visited.
BAS, yes, he said as much. Being rubber, they will have to be replaced. They may pull through the existing conduit or just install new PVC cables under the floor.
Oh, I'm certain the old box must have been there well before we came here 10 years ago. I wouldn't be at all surprised if it was put in when the house was built in the 1920s - and maybe even on a DC supply! I don't know how early AC came into use around here.
If they replace any fuses in the old box with solid links, won't that mean that the (new) connecting cables to the meter/CU won't be protected?
Taylortwocities, he did measure the earth resistance(?) which came out at 0.34 (Ohms?). He assures me that this is safe, being well under the 0.8 limit. Looking closely, there is a bare earth wire from the supply end of the conduit to somewhere behind the old wooden board. I shouldn't be surprised if they replace that as well. He did mention something about digging up outside as well - presumably that will involve our drive.
Guitarguy, the old rubber cables at the meter/CU end do look outwardly at least to be in reasonably good nick but, as you say, will need replacing anyway.
That's what surprised me: the fact that, not only is the old box under some loose floorboards (which may have been acceptable at the time), but that in the meantime (long before we arrived) a fitted wardrobe was installed directly above the area. I know nothing, but I would have thought that the old box should have been accessible. It is now anyway!
The DNO gentleman told me he'd be submitting a report (and another photograph) and that the people who do the remedial work will be in touch, although they tend not to rush too much unless it's a dire emergency. He advised me to call them tomorrow or Wednesday to see what's happening, and to tell them that my wife is worried sick. B***er that, I'M worried sick!
Good news: they will pay for it all!
Again, I can't thank you all enough for all your interest in our predicament.