The missus had mentioned that something we do upstairs at work makes the
pipework downstairs clunk, it clunks semi-regularly when there's nothing
being used.
We've only a loo and a sink plumbed in upstairs so I had assumed it was
when the loo stopped flushing.

But I was at her desk the other day, shop shut, heating off, nobody
upstairs and the pipework let out a decent clunk.

Short of a better idea I wonder if it might be something external.
Is that possible?

Because if not I need to investigate whether something is plumbed into
our supply that shouldn't be, we are on a water meter.