zzapper writes:

[Working on cistern]

Thnx for above. The unpleasant aspect of above is disconnecting the
water supply feed, which means switching off main tap.

Me thinks alas that this is a job for a plumber.

The water regulations say that there should be a servicing valve close
to the cistern. If you haven't already got one and decide to call in
a plumber, you might like to consider having one fitted in readiness
for next time (hopefully in the distant future).

--
Mark