Thanks again for that input guys. Yes, my working fixer hangs around for a few sessions or more, I usually end up just splashing a little extra stock in I feel it's getting a bit tired, then replace it after that session. I had a great collection of these concertina bottles a number of years ago, in the darkroom I learnt in nearly 20 years ago they were all we had, but then I found out about their shortcomings! That's why now I only use them for working solutions that are only going to be in them for a couple weeks, and my dilute selenium, figuring it should last well no matter where I put it. My current batch has been in a concertina bottle and going strong for over two years, with jut a little top-up when times get too far over the 3 minute mark.
Good to hear of stock fixers lasting a while, I had always assumed (I think the Ilford data may state?) that 6 months was the limit for opened bottles, and one week the limit for working solution... Or maybe I have just always err'd on the cautious side.