I have managed to remove the stock nut using my trusty scalpel, just cutting away the glue between the fretboard and headstock side of the nut and then lightly tapping the point of the blade in between the bottom of the nut and the fretboard has loosened it enough and it popped out with minimal damage (only casualty was the point of the blade. It took a bit of patience but i've seen some bad results from people trying to knock it out with a hammer and screwdriver (the most common solution I found on the internet).

One query which I can't seem to find via the internet - when I place the bone nut in, it's angled towards the fretboard slightly (third pic below). It appears that the string slots are parallel to the fret board it just pitches forward a little. Aside from the aesthetics (and mild OCD) I just wondered if this was normal for bass guitars and won't impact intonation etc.?

Thanks as always for everyone's help!