Ian Keeling wrote:

Just to confirm - this specimen is approximately four feet tall ?

...
though probably
not as large as it ought to be given its age, considering the size of
the pot it's in, which I rather suspect is the original one...
...

It's simply that it's being foour feet tall might seem to conflict
with an obsevation that the plant could still be in its original pot.
As presumably its girth would have expanded at least to some
extent in the interim. Unless it was grossly overpotted to start
with for some inexplicable reason.

Also, if it is indeed 4ft tall, then any approach to repotting might
need to be modified accordingly. Clearly tongs cut from old bath
sponges are out of the question for a start.

Thanks to all for the useful advice.

Yes, it's certainly at least 4 feet tall. The pot may not actually be
the original, but it is certainly way too small for a plant that size -
I'd estimate (from memory) that the pot is 5 inches in diameter
(possibly less). I'm pretty amazed that it has survived at all really
given the extent of the neglect that it has suffered.

The only certain way to kill an established cactus is by over watering.
I have known big plants flower after a whole season with no roots at all!

Be aware that repotting will encourage it to put on a spurt of new
growth. There might be a case for keeping it slightly underpotted.

Regards,
Martin Brown