I tried to have a look last night, from a window in the first floor of the house, at what was behind the garage. It seems that there may well be enough room to stick a small camera or mirror down the gap between the roof and the concrete bit. In fact, I think this might well be Dave's first task on his return (5pm landing, so perhaps a bit optomistic. Otherwise i might climb up myself.)
As you can imagine, since finding the tunnel I've also been trying to speak to people who might know a bit of history about the house. Interestingly, someone who used to work as a gardener for the old owners, told me that they had told him a long time ago (i know, i know!) that there was an underground chamber, but in a different location from the one we now know about.
So..., just in case thats not a tall tale, I also think that I'll start by checking under all the manhole covers (there are a few).
Lastly, WW2 historians are visiting this evening as well. We can hopefully start moving things around once they've been.