What I found very odd while we were digging that hole was that one side was dry and we had quite deep but when we widened it slightly it suddenly got wetter and small amounts of free water began to puddle in the hole. Be interesting to know the salinity of this water.
The hole is in an area where two weeks before it was too muddy to walk.
The bad news is that, to me, this means the "water table" is getting smaller so even though it is not far below the surface it is only just below the surface in a smaller and smaller area.
Personally I don't think its strictly the water table. Under the silt appears to be a pretty consistent layer of clay which means water isn't coming up from below. I'm guessing it is coming up from specific gaps in the clay as "springs" and a slight slope in the clay bowl towards the old hedge and also towards the first flash is causing it to drift in that direction through the more-permeable silt layer.
I understand there used to be a channel from the third flash and this ran roughly down that direction as well. So there might still be some water getting through the silted up channel and be "running" along this slope and helping fill the hole on the way. The third flash seems to be keeping its level pretty well so suggests that it has a good spring in it and/or gets a lot of any runoff from the surrounding land.
And therein lies part of the solution IMO. Reinstate that channel and the third flash can keep topping the levels in the other flashes and reduce the impact of drought. Go further and extend and/or deepen the third flash and it can act as a reservoir.
Of course a clandestine syphoning of the sailing res would be much more fun
Paul