I looked up Jade plants and it definitely looks like one. The leaves on mine
are smaller and rounder than those I've looked at, but I'll go with Jade
plants - thanks everyone!

In terms of it's disease, the only thing which I came across (which may have
affected it), is that they can actually get too much light and there was a
mention of stem rot as a result of this. It hasn't complained since I put it
there earlier this year, but perhaps the spring light (which it hasn't
experienced in this position) is a bit intense. I was out there at around
midday and realised that I was squinting very hard. The glare of the light
was almost too much for my eyes.

I cut off a few stems and they were fine on the inside, just this ugly mess
on the outside. It I didn't look closely at the areas which looked as though
holes were forming, so sorry for anyone who may have been interested - I was
just keen to get rid of the ugly alien bits and not look at them too
closely.

I've confidently cut the whole thing right back. I wasn't happy with it's
shape anyhow ;-) Although it gets so much light where it is, it's so far
grown to be fairly spidly, maybe I was giving it too much water? I've seen
big strong bushes of this stuff, I guess the conditions will determine the
weight and the shape.