Not for definite here, but if there was a root ingress into the sewer pipe
and you've been hack at the roots, you may have dislodged the sewer. This
is highly unlikely, but it could happen.

Well hopefully not! I do know that the house next door but one had a huge
sycamore tree that had grown into the sewers and the council had to some and
sort that one out - I dont know how far down they went though.

The tree sap that's lying under the tree is creosote, that is what
creosote
is extracted from.

I didnt know that so that may be cause.

When you saw the water flowing into the hole last week, was it after a
heavy
downpour of rain, even on the day before you went back to the hole ?

It had rained but not heavily that I know of. I guess it could just have
been ground water, I was just surprised at how quickly it was flowing - I
would have expected an 'oozing' rather than a flow (but we have so much clay
that I suppose you can get stores of water quite easily). I havent seen it
since, but then again I havent dug in that area any more :-)