Thanks again Jonathan for your good advice!

How much have you found that spindle mount for? I could probably get a block of aluminium and bore it for you for less money...
Well, I can get 2 spindle mounts for about £70 shipped, plus taxes if they stop it. I was just trying to estimate how much it was going to cost to have machined from a block and judging from recent work I've had done, ALOT! But if you could do them for less, then obviously that would be a massive bonus! AdCNC has offered to do them for me, so I plan to see him this week and have a nose at his machines while I'm there :)

Regarding the shaft, I will get this done professionally (i.e not by me) because like you said it needs to be a good fit and screw cut. My Idea is to have a flange at one end to bolt the ballnut to, then a locknut at the other to pull everything together, then the pulley after the thread on a slightly smaller diameter shaft.