Thanks for all the input guys, appreciate it. I've not been so worried about the cost of wasting chem, since as you say, it's cheap, and particularly fixer. It's more just that I'm tight (good Scottish upbringing!) and don't like to waste anything.
I had wondered about the concertina bottles leaking, at the moment I only use them for my dilute Selenium and working solutions of fixer and paper dev, which might last a few sessions. And a small one for fixer concentrate too. My stock developers both live in large bladders, and store really well.. Maybe I'll have to get another one to pop fixer in and just start getting the 5L bottles.
Thanks again for the comments.