I have decided to use this chain with these gears,
I will use the 15mm gears for the X and Y axes and the 40mm ones for the Z axis. For my mini Darwin I will need: Y axis - 510mm x 2; X axis - 564mm; Z axis - 972mm. This gives a total of 2556mm so I will need to order 3m.
This works out quite extensive at £30.25 for all the chain and gears but I will have plenty of gears left over (they come in a large pack) and will be happy to send any to anyone in the UK who wants any.
The motors I have are 200 steps per revolution, using a 15mm gear the axis will move 47.115mm per revolution. This gives an axis resolution of 0.24mm which I will be perfectly happy with.