Yes, good point Mab, better stick to metric units.
It's estimating the cross section that's hard Chris - it varies so much. Will have a go at three or four places and then average it out.
At the moment I am much put off by the requirement to have a pipe running down by the side of the beck - too ugly. I had naively thought the 'thing' (turbine/wheel) could go in the beck itself or in a dug out side bit, ie the water came out of the beck, through it, and straight back into the beck.