Sell Gill?
If that's right, it's an extremely interesting survey as it shows the 1990s CPC "re-extension" beyond the former downstream sump was still well known about in 1941. Perhaps the war years are the reason it seems to have slipped from everyone's memory, as folk had far more serious things to think about in the early 1940s.
Perhaps a Swallow Hole?Somewhere in the Lakes, or possibly the Broads?
... Graham is nearest so far!