A few people seem keen to report their horticultural findings and experiences, and I've no idea how far back to trawl to dig out the last thread on the subject -so let's start a new one.
OH ordered 2 x 200 litre water butts and linked them together to catch the rain from the garage roof. (Two already in place by the shed- small roof so catches little rain).
After 2 days of only moderate rain the things are absolutely full! Have transferred probably 20 gallons of rainwater from the new into the shed water butts. Yay!
Will now probably have the wettest summer on record. But at least my bag-grown potatoes will not go thirsty.