On Fri, 07 Sep 2012 22:12:01 +0100, Graham.
wrote:

The bangs however seem to emanate under the floor at the back of the
house near to, or maybe even beyond the party wall. The elderly lady
next door says she hasn't noticed anything.

She would say that.
What she's not telling you is the basement and attic are full of dope
plants and the thud-flicker are when the contactor for the lights cuts
in and out via a timer.