Hi Gary,
Has there been much investigation regarding the springs in the village of Brompton-by-Sawdon? 10 miles east of Pickering, and very similar in nature to Keld Head, which again must be draining the higher ground to the north. Definitely on the Jurassic Limestone boundary. I imagine it would give that depth requirement you're looking for. Two separate ponds, the fish pond and the mill pond, fed by springs that form the head of Brompton Beck.
Thanks,
Phil.