Just a quick update to say: problem solved!

It was a vacuum leak, which got worse, and therefore easier to find. There are a couple of vacuum connections on the inlet manifold which are not used and capped off, these caps had got brittle and started to split, split got worse and I was able to pinpoint. Just replaced both caps and now have a nice smooth idle.

The throttle cable was sticking slightly as well, so I replaced that too.

Thanks for all the helpful suggestions.