Hurrah! After a fair amount of hunting round, we have found our droughtplug set up (a hose with a rubber bulb in the middle which can be pumped to draw up water and establish a siphoning flow). With the local climate being so atypically dry, we need to get back into reusing more grey water on the ornamental parts of our garden.
We had already been saving water in the kitchen (from light washing up and rinsing lettuce and radishes from the garden) in a bucket at the back step but being able to send a bath of water downstairs (not typically a whole bath but what we capture by leaving the plug in when showering) will give a good boost to what is available.
Just as well, too — in my continuing studies for my RHS Certificate in Horticulture, I have recently learned that plants release about 500 molecules of water for every molecule of carbon dioxide they take in to fuel photosynthesis and that 97-98% of the water they take in via their roots is lost by the process of transpiration (although it does cool the leaves and maintain the water column to to the top of the plant on the way out).