Long time reader but not posted before, but I have some information pertinent to this thread;
Agaricus bisporus; that is a Tallboy crater - there is another just to the West of that location.
The Grand Slam crater was considerably larger than that (130ft diameter 70ft deep) and was filled in at the same time as the submarine pens were covered over when they tidied up the range after the war. It is just to the North of your Tallboy crater which i've marked on the attached map with a black circle. They were aiming for the submarine pens..
I've spent hours at Ashley Walk and have just about located all of the remaining artefacts and positions
Cheers,
Bryan