AbThCl is quite right. You'll rarely be offered a perfect 500x20 strip down the middle of a hundred acre field. Farmers are always looking at ways to make their field operations more efficient, and if they can get rid of a curved headland (field edge) as that sits alongside a potential strip site, that's the best way to do it.
So, whilst you might want 10,000 m2 (2.5 acres) for the strip, you might actually find that he takes out 4 acres to make his life easier. He can then enter the spare unused land as 'Field Corner Management' under the new Mid-Tier Countryside Stewardship scheme and get paid for not farming it.
The more I think about this, the more I realise that unless people have farming knowledge of all the various schemes, it's a tricky approach to make. I know of at least ten places on my land where I could put a strip and be financially no worse off, but that's only because I know how to mitigate the loss of crop by using various grant shames to top it up.