So today I traipsed around some supermarkets looking for a power bank. Lots of empty hooks in the first two, with only the bigger and more expensive ones available. Next one was Asda, where I bought an Onn 3350mAh for - wait for it - £4. Photos are below.
It is 10.5cm (4 1/8 in) long, and weighs 79g (2 3/4 oz). Usb and micro usb sockets are in one end and there's a flush switch on one side. One press for on and two presses for off (I found out the hard way). When it's on, one of a row of four little lights on the end shows blue, I assume these will all be on when it's fully charged. I did find a "manual" for it online but it tells you beggar all, it's not worth saving.
Now don't all howl with derision. If I get one full charge out of it when needed, it will be good enough for me.