Hey guys, thanks for all the replies.
I changed 50% of the water so the salt level should be back down to tolerable levels again. Too late though - the nitrites are going up already, sigh. Though there is only a very slight increase (not even 0.25 yet).
Alex love: you know your wonderful shop?

I don't suppose you could do me a huge favour and send me a salinity testing kit like the one you recently bought could you? I'm fed up with not knowing exactly how much salt is in the tanks. Please let me know if that's possible.
However, just to make everything 10 times worse, I now have a problem with the pH in his tank - I tested it just now and it was somewhere around 8.4, possibly higher. Bet he just loves that, not.

I think it's my old friend the CO2 at work again, but Augustus's tank has virtually no surface agitation, so technically if the CO2 isn't being driven off then the pH should be down, not up, due to the carbonic acid it produces. When I did the water change, it dropped to 7.8 (now that IS the CO2 at work), but I just don't know what the core problem is here.
I'll think I'l be lucky if I manage to keep this poor little fellow alive frankly.
His tail is no worse but no better either. And no, unfortunately I can't get either Jungle Fungus or Bettafix. All I have is Melafix, which I already added (though only half the recommended quantity because he is so small and fragile and I know it can be harsh). As for antibiotics, I have a little bit of Maracyn and Maracyn 2 left, which I just know is going to crash the cycle completely because it always does. Would Maracyn be any good for a betta? He won't eat Medigold (tried it, not that I had much hope on that one!). I suppose I could try a UK antibacterial med, but I've never had much luck with them. Drats! I hate it when my fish are ill and I don't know how to get them well again.
B/Oranda - salt will actually cure finrot, as long as it is not too severe, as long as no internal infection is present and as long as the water quality is perfect.

Any other thoughts guys?