Great info, thank you. That one looks very much like it'll work. Thread size is an unknown without being able to get under properly but for the low cost I'll just take a chance and order I think.
@Allanm I will try CAK tanks, thank you.
I think CAK tanks do the same one I linked to. I looked on their site and they have this one
http://www.leisurelines.net/new-male-cap-for-tanks-mtc-354-p.asp
which doesn't give a size, so I googled the product code which came up with the other one that I linked to.
(I know it says leisure lines, but that is cak tanks online shop)