Sorry, I missed the 'flat' bit - and assumed you had access to downstairs.
Do these pipes feed anything below, or not. If so, what I said is ok - if
you negotiate with the person below.

We're on a separate water supply to downstairs. :-)

To be fair that's a recent development - it was shared with downstairs -
what a nightmare that was by the way... we actually have water pressure
now - woohoo!

Andy