Just a little bit more information, to avoid possible confusion, the downstream sump I'm on about is the spray-lashed puddle directly under the waterfall at the very bottom of the pitch. It may not be entirely obvious in wet conditions - or it might appear at first glance to be just a small pool or even an upstream sump. It's only in drought that it swallows all of the water descending the shaft - then you can tell it really is a downstream sump.
When you've unclipped at the pitch base, if you walk off downslope, the stream disappears off on the left after 20 m or so into a very obvious downstream sump pool. But this sump is only about 7 m long and you can get to the far side of it by taking a dry passage on the left a few metres beyond.
You meet the stream in a tiny chamber flowing from left to right into an immature passage, The Lancashire Caving & Climbing Club spent several weekends trying to chisel this open in the mid 70s, without success. I think it's partially blocked with gravel now but a tight squeeze was passed here to reach the top of a small drop, choked less than a metre down. If the water was dammed here and then released, a loud waterfall could be heard. However, this unattainable waterfall may have been entering the Cambridge Uni extensions, at lower level, gained by bailing a tight duck in a different place which led to a 15 m pitch - so there may be little point in giving the LCCC dig further attention now we have an idea what may be beyond it.
The destination of any water sinking into the small sump pool directly under the main wet shaft you've descended is unknown - and hence is of interest as a possible way on in the direction of the risings in Trollers Gill.
Whilst I'm on with this brain dump, the big area of limestone to the east of Trollers Gill (opposite side from Hell Hole) also has potential. Mines in the Burhill area are recorded as causing the water emerging from Nape Well Cave to flow murky. This cave also ends in a shallow sump, which has been pushed very hard by CDG members over recent years, reaching the point where further progress was pretty much impossible by diving. But this sump could also be emptied in drought (probably by pumping, rather than siphoning). The way on is a partially backfilled rift with small airspace above it on the left, just before the end. The rift could be dug fairly easily if the water was got rid of.