A difficult one, as a) it was 45 years ago, and b) the area has changed so much!
Looking on GE, I estimate it to be where Watermead swings round.
If you project Watermead east, there is a track on the East bank which I'm sure used to cross a brick built bridge, and the unloading point was on the East bank, just North of this bridge. An old Nicholsons or similar would pinpoint the bridge more accurately.
The unloading point was just a low piled wharf, and a 360 was used with a grab.
Initially, we used to wind about 2-300 yards before the bridge and reverse down, through the bridge, but it was a complete PITA, and the 360 was utilised to dig out a winding hole just North of the wharf. GE shows no trace of this now, but it is incidentally where the 2 "oversized" boats were. They must have been there when I was, but I can't honestly say for certain. I know they were there in 1973 when I took "Chertsey" up there on a holiday trip.
Next time I take a tacho break in the area, I will try and nip down for a look!