Hi Neil, sorry, been out all evening with a customer designing a system for him.
I use 100m of 8mm minibore on each reel -the outside diameter (o.d) is slightly bigger than microbore, I think its about 13mm (it pushes up into standard half inch hose really nice and tight). To be totally honest, 100m fills the reel a little too much, 90 or 95m would fit much better, it really only needs a little taken off it. Even completely wound up onto the reel the water pumps through it really really well. Best stuff out there.