If it were me, I would just chop of the top section, root it and discard the rest..

Dunno what's causing the brown spots....might be scale. Try wiping the brown spots off with a cloth dampened with rubbing alcohol, if they come off without hurting he plant, its likely scale. They may also leave a reddish splotch on your cloth when they get squished. read up on this Forum on how to get rid of them.

Thank you! The brown spots don't wipe off with alcohol - but I have just picked one off and it came off cleanly using my fingernail, although the centre of the spot, is a little hole into the flesh (see photo - towards bottom of pic). So do you think the middle (spotty) section is beyond saving - I shouldn't plant that section? (either attached to, or separate from, the healthy top section?). I'll need to google to see how to root it (I've never had a cactus before) - any tips?

The way that comes off makes me think it's scale, but I don't have enough experience with that parasite to have seen it behave that way. The discolored circle around it also came off, which must mean that each of those blotches is a stationary scale insect surrounded by a puddle of its own, dried honeydew (bug poo). Eeeew. Maybe scale insect can sit in one place on cacti because the flesh is so juicy. Blech.

Do do you have other plants? I'd be concerned about it spreading to others. Take care of it quick with a good alcohol wash. You need to get rid of eggs too, but I don't know how you do that.

Yikes! I will quarantine it. I have lots of succulents in that porch.

Thanks - might be I'm best to discard the bottom and middle sections and just start afresh with the top bit.

Yeah-the circular things look like scale to me. If you're brave enough, take a Q tip and dab each little spawn of evil with some rubbing alcohol. Then once you've gotten rid of all the ones in sight (or if you feel too queasy), spray the entire plant with 50/50 water mixed with rubbing alcohol.

Good luck!