We've been flushing with rainwater these last few months (until the recent long dry spell!) with a cheap 12V pump in a water butt, float switch in the cistern and avoiding modifying anything in our rented house by shoving the wires and fill tube up the overflow.
All new houses should have systems like this installed as a matter of course - it is just so wasteful (& expensive) to use drinking quality water for flushing the bog.
Another thing to add to the list for the long-awaited self-build.......
My old man rigged the downstair loo up to a water butt with gravity feed. He also rigged a caravan style pump up to a micro switch as back up because the gravity feed is a bit slow.. The caravan water pump is powered by a battery charged from a solar panel. I dont think it cost him many pennies to make as he has most of the bits.