I'm presuming the problem is that the rain butt is vented to the
atmosphere at the
top?
Simply replacing this style of "cistern" with a sealed "tank" would
solve your
problems?

Or perhaps do as I did: sat the water butt on a high wooden platform to
make use of
the wasted upper space and to give me lots of flow pressure.
If anyone wants the dreaded photos, drop me an email!

Mungo :-)