If it's where I think it is, and I'm pretty sure it is, then I went there several years ago on an evening trip with a mate who'd been digging there on an off. We siphoned the water out of the sump, which took about half an hour, and then went through so I could be shown the dig.
The dig was in a tube-shaped passage about a metre high; the bottom half was full of mud, and a thick layer of flowstone had formed on top, leaving a desperately tight gap over the top. The diggers had adopted the approach of digging out the mud from under the flowstone, and then bunging in a car jack on top, and cranking it until a chunk of flowstone broke off, then repeating. They'd given up when it got to the point where the jack was breaking instead of the flowstone.
Now, it was just possible to wriggle into the top of the passage for a couple of body lengths to a 90 degree corner to the right, where the gap got smaller, but beyond was an echoey black space (probably the top of one of the avens a little way back downstream
Having been persuaded to go in to look round the corner, on my way back out, I suddenly found myself jammed. Stupidly I'd gone in with an oldham brick on my waist, and the bloody thing had dropped into a shallow gour about a centimetre deep, and now I couldn't go forwards or backwards, I couldn't contort my arms down to sort out the problem, and my mate couldn't reach either cos my legs were in the way.
For the first half hour or so we had a jolly good laugh, all at my expense of course, until we realised it was getting close to last orders, at which point we realised the situation was serious. After another quarter of an hour, I was going to send my mate out to look for a long stick or something to prod the oldham box with, when finally, after lots of wriggling about I felt the battery box pop up out of the gour, and after a further short struggle against the buckle of my battery belt, which managed to get caught up on the lip of the gour, I was out, and racing for the pub (we made it!).
Anyway, that's what's behind that sump. Lots of interesting little bits like that scattered around Giants, perhaps somewhere there's still some that go, waiting to be found, although lots of people have spent a lot of time looking very hard.
Duncan