So, one of the problems with the Mockingbird was that the nut was much too high. I tried carefully sanding the nut shelf but it wasn't enough and I made shelf convex, which is obviously no good.
There was absolutely no way I could get this flat enough with hand tools, I just don't have the eye or skills.
So I made a jig. This was a bit of acrylic sheet cut up and stuck together so it's snug on the neck and resting on the outer edges of the fretboard. That way it should be parallel with the fretboard and pretty solid. I was thinking about making a clamp for this but just opted for some cable ties as it's not like I'll be using this very often. The jig isn't very pretty as it's been roughly cut, but I wasn't aiming for pretty.
A full size router is quite heavy so I used my Dremel with a basic plastic router attachment. I used another piece of perspex with a decent straight edge clamped to the jig to ensure I couldn't start chewing up the fretboard south of the nut.
Working carefully back and forth this did a great job of gently shaving the nut shelf as flat as I expect to see such things. In the penultimate photo it looks like I've missed a bit, but it's actually a 'tide mark' on the end of the fretboard where there's no finish.
Once I blew the dust off, the new gold nut I'd bought for this guitar fitted perfectly on, I had previously been very hesitant about this job but it's turned out perfectly. I ended up shimming the nut very slightly with some card but I deliberately cut the shelf quite a lot as it's easier to shim the nut than keep going back to take more material off.
With the new nut in place and the guitar restrung, the nut height seems spot on. It's a bit late to play it now but tomorrow I'll have a good play of it.
Of course all this will just show up the fret wear, but I've made progress and perhaps dressing the frets is the next thing I'll learn to do.