So, I've ordered a new nut for the Columbus, but in the meantime while I'm waiting for it to come I've removed the old one.
Standard wisdom is to score the finish round the old nut so it doesn't take it off then give the nut a quick tap with a wooden block. This worked just fine.
There was quite a nasty build up of finish in front of the nut and the new nut is going to be slightly thicker than the old so this needs to be flattened down. It was also generally a bit crusty around the nut.
Sadly, with the very first stroke of a needle file a flake of finish flew off. I managed to find it and stick it back in place with a drop of superglue. If it were a valuable vintage guitar obviously this would be a disaster, but as this is a cheap shitter it really doesn't matter.
I then worked at it with some sandpaper, a needle file and Stanley knife blade to clean it up and it seems to have gone OK. There is a little spot where the finish has gone through but it shouldn't show once the nut and truss rod cover are back on.
So, just need the nut to come now.