Thanks for the feedback guys! Enjoying seeing others builds come together. Xmas seems like a very productive time
I lowered the height of the nut earlier on today, only took a couple of gentle taps with the hammer and a block of scrap wood and it came away cleanly.
Took about 2.25mm off the bottom, used a wood block to hold the face against to keep it nice and square. Masking tape helps line up how far you need to take it down, and the excess tape gives you something to hold.
The underside of the plastic nut is hollow, so I'm glad i didn't try to lower it by cutting the slots deeper, that would have probably been a bad result.
A couple of minor tweaks, and it's now playing really well. I can hammer on on the first fret quite comfortably on all strings now, and the overall action and intonation is consistent.
Have been thinking about what I might build next... A TB-4 would be cool, I'm thinking a different, edgier take on it...
That can probably wait a few months though, best not to encourage my new found addiction to building these things!