Pitlamp said:

Thanks, is that "-44" as in "44 m below the level of the entrance"?

Really what I'm interested in is how far above sea level the water surface in the Lake is normally.

Mr Oldham's Concise Caves of Devon, gives an elevation of 200 feet, which I assume to refer to the entrance

also a vertical range of 50 feet (which I assume is to the water) or a vertical range of 170 feet if diving.

Not sure how helpful that is.

The other thing to consider, apart from the construction of the bypass, is that the river is very close to the cave (although on the other side of the dual carriage way) and will almost certainly have some effect on the water levels in the cave...