Meanwhile in the plastic water skin (refillable, flexible bottles) I tried the similar-to-CWS-recommended Platypus bottle: https://www.amazon.co.uk/gp/product/B008W0I71E/

but after a couple of months it developed a small leak - which, when pressurised by trapped air at altitude, nicely watered the inside of my carryon

It is made of quite rigid plastic and folding it in a bag seems to lead the seam weld to fail at the point where it folds.

I have replaced it with this one: https://www.amazon.co.uk/gp/product/B00AXYDRT4/
also from Platypus but which is made of a significantly softer and more flexible plastic. So far it is holding up well after 6 months. As well as not leaking, it folds up smaller when empty.

Via the carrying handle, it doubles as a tripod weight for photography though only at the beginning of the shoot, at the end it's empty and useless