but I have been reusing it for 10 years
heck! Ilford aren't going to get rich from you!. No idea why it has suddenly turned, but I think the best way to use fixer is to keep it neat, make up working strength to the quantity you need = 1l or so, then keep track of what you have used it for combined with "clip test". When the fixer clearing time reaches twice what it was when you started, discard and make a new batch of working solution. I find my monitored capacity is a little under what Ilford suggest, but there again at twice clear time from fresh, there probably is still some capacity left. Fixer isn't so expensive that I am tempted to stretch it to the very last sheet.