This is one of my 10 year olds, (I don't have any species ids on my old ones yet!) it was in a very small pot which I recently had to smash it out of. I
thought it may have had root rot or fungus, I fluffed the roots out a bit, re-potted in good soil and have him propped up on a crate to get air in underneath. I also applied some Yates Anti-rot to the soil only. He seems to have revived slightly as a flower has popped up but there is still some badly 'scabbed' areas at the base. Any advice please? it is an enormous old plant and I really want to save it, I'm just very inexperienced with this sort of thing

I'm not completely sure, but to me this just looks like the corking that occurs around the base of old plants. Is the scabbed area soft and mushy or scabby and wood-like?

Wonderful! I just had a good look, and it isn't soft at all.
My others which have started to cork (which I just discovered) look completely different, the area is brown and woody. It was the black part that was worrying me. Thanks for your help

Don't thank me just yet... that black spot is my only area of concern. It's probably nothing, but I'd definitely keep an eye on it. If it spreads or goes soft, some aggressive treatment may be necessary, but at this point I think it looks okay.

i had to do some major aggressive treatment on my old golden barrell and i mean aggressive but it still seems to be hanging in 3 months on. it now has 3 massive holes in it one is about 10cm across and about 30cm deep but i did manage to cut all rot out and then smother it in sulphor. fingers are crossed!! mine also has the exact same corking effect around the base but no problems with it at all.

Thanks guys, I'll keep an eye on the base and see if anything spreads, it's been about three weeks and no spread since I took it out of the old pot so far. I thought the first flower in ages was a good sign.
Darren I am glad the old Golden barrel is hanging on! I nearly cried when I saw that