My mum lent me £20 so I was able to buy them on Friday :).
Order was dispatched on Monday and arrived yesterday - excellent service, thanks!

Been doing some testing...
I've not got round to making pulleys for a 1:1 ratio, so for the moment the closest I can get is 22T on motor, and 42T on screw.
The motor stalls above 1250rpm (no load), yet it still has enough torque to drive the X-axis at 25mm/s (1150rpm). Trying higher feed-rates than this gets a bit dicey.
I think this means that if I put a 1:1 ratio, then above 25mm/s is almost certainly attainable since the motor will only have to run at 600rpm to get the 25mm/s. Having said this the current speed is pretty scary! I had to put a lot of tension in on the belt to stop it slipping.

Currently I'm using 1/4 microstepping. I've tried 1/8 but it doesn't seem to make much difference. What do you recommend I use? The computer I'm using is only 1.4ghz, this might be a limiting factor? I'm using Mach3 at the moment, I intend to change to EMC.
Also, I've set the port frequency setting in Mach3 to 25Khz. I tried 35Khz but it seemed worse, should one be better than the other? I've set the pulse width to 5us, is there any reason to change this? I guess not unless I start using 10000pulse/rev...

Thanks for all the advice so far.