I don't think the lack of progress is for want of trying. I've kept going back and must have wandered over every inch of the area. There are little sinks round the flanks of Bole Hill and Withered Low that must enter the system, but they are rather small. The most promising one I found was near Central Cottages at the head of Flag Dale (approx 1108 7436). It can engulf a massive amount of water in flood conditions, and it never overflows down the field.
There is an old concrete structure of doubtful purpose that the water seems to flow into, but it isn't seen again. Right on the field boundary you can sometimes hear water flowing underneath, but getting to it might involve digging where you can be seen. You are unlikely to get permission because I think it is owned by the company who is quarrying Great Rocks Dale away. Locate it in the winter and dig in summer? It is in a jungle.
Bull Ring Cave would be a good one, simply because it is there! Any lead in the area is worth following.
I got involved briefly in the dig at the crossroads in Dove Holes before the old lady put her foot down. It is a very promising site if permission could be regained.
One day someone will find a lot of cave (but might end up in the railway tunnel along the way). 150m vertically and just over 6km from sink to resurgence!