On Saturday, 17 February 2018 18:51:48 UTC, Gareth's Downstairs Computer wrote:
Having made a boo boo and overordered some replacement
bathroom shower hoses, I wonder what is the screw thread such that
I might turn some mating pieces to join them together as
a form of garden hose?

arf inch bee ess pee.

Such couplers are already made eg
https://www.ebay.co.uk/itm/111718270192

Owain