Thanks Kyo - really pleased with the design.
A couple of points I picked up on while building:
Pay the little bit extra for the M5 tapping to be done on your extrusions. OMG that takes forever. If you're buying in the UK, Ooznest are a good supplier, I've found their custom cut profiles to be a little pricey, but accurate and square.
When assembling the wheels, and mini-v wheels, it doesn't say that you need to put one precision shim on the inside, between the bearings. In hindsight this is obvious so you can tighten everything against the inner races. I won't say how many I made before I figured this out :-)
The wheels from either Openbuilds or Oozenest are horrifically expensive. Standard X-treme wheel kit from Ooznest will cost you £6.07 for each kit, and £5.02 for the mini wheel kit (and you need 16 of the big wheels, and 24 of the mini wheels). Look on Aliexpress at a shop like Xinfact, similar thing but for £1.40 per set. https://www.aliexpress.com/store/product/Poly-Carbonate-Solid-V-wheel-Kit-for-V-Slot/123598_32315197095.html?spm=2114.12010618.104.22.1681d05e96ZGMOq
Chris Laidlaw's plates are spot on (thanks Chris) - I can also highly reccomend his mount. A little bit more expensive, but really nice. I went with the taller plates for more z-clearance when milling larger items. If you go for the taller plates, you'll need 2 x 18mm MFD base boards, and a 12mm MDF board to make up the gap so you have a 'sensible' cutter length sticking out of the collet. This isn't a bad thing at all, the 12mm board is easily replaceable if you mainly use double sided tape, and you can put a grid of M5 tee nuts into your top most 18mm board for a lot of flexibility when gripping other, taller things.
Getting hold of a 40T Z axis leadscrew pulley isn't easy (at least here in the UK). I've gone with the 20T one from Ooznest (http://ooznest.co.uk/GT3-Pulley-High-Torque-20-8mm-Bore?search=MC-GT3-PLY-HT-8). Be sure to set your GRBL settings accordingly.
It's a little bit unclear as to whether to connect the ENA- on all the stepper drivers. The Phoenix CNC wiring diagram says yes, one of Kyo's electronics overview videos says you don't need it. I connected it up to be on the safe side, but if you do connect it, the machine won't move. Do not connect ENA- on the stepper drivers :-)
I've also posted a set of STL files from my build in case anyone finds them useful: https://www.thingiverse.com/thing:2679259
Hope this helps others, and again a huge thanks to Kyo for the massive amount of effort he's put into the design, and the documentation.