You could try plastic water pipe and use the various fittings for joints
but if it is not properly anchored you will lose it on the first windy
day!

That's not a bad idea, I think I'd only need 3 lengths of piping. It would
be cheap and easy to drill holes in (I would screw it securely to the
ground/wall/house so it wouldn't blow away!). It's not going to be too big,
and it's in a corner, so hopefully it will withstand the coming
global-warming fuelled tornadoes etc! I suppose if I was really clever
(which I'm probably not!) I could make it with a gutter to collect water in
a water-butt . . .

Thanks for that one!

Andrew