Ploughing through all the old details of previous planning applications there was mention of a "system of undergroundwater tanks" which harvested rainwater and then using a wind generator supplied water to the growing tunnels that were there in the 1970/80's or digging further back (sorry about that awful pun!) there was a septic tank nearby in the 1950's. I think another visit to the planning department maybe necessary to see if we can locate these items on a map. But for now definitely a case of filling this one in. The frightening thing is that there was no cover on this one and it was just hidden by all the excessive vegetation (weeds, bramble etc.) A miracle that no one has fallen into it.
At the end of day 2 the site now looks like this: