At least I'm not the only one confused by this!
I've dug out and studied the plans for sewage to our house and nothing goes anywhere near where the chamber is. However, the plans also fail to show a manhole cover behind my house for the sewers (which I know for a fact is there) and a disclaimer at the bottom of the plan basically says "these may be inaacurate but we (the water board) cannot be held responsible". All in all, it was useless.
Penny, you are quite right- I have yet to reach the bottom. There is a fair bit of earth to shift (my wife won't have anything to do with the hole so I'm on my own) so it is just a case of having the time to do such a thing. I have already discovered that the earth goes down at least another 2 feet. I suspect there must be a good ton of earth.
Richard, I will try to find out from the local parish some history. I know from brief research that Wainscott has quite a varied history to it.
Keep those ideas coming!!!