I was wrong! It was not Sarsons malt vinegar that I bought but Hong malt vinegar, now that I have looked at the original 20 litre container, claiming 5% acidity. It does indeed now contain tasteless watery brown fluid. So I apologise to Sarsons for casting aspersions, and will certainly not buy Hong vinegar again..... A false economy. I made very good chutney with it (in my opinion) last year, and used it for window cleaning, wiping out the fridge etc, but it is absolutely useless now.
I am inclined to agree with the advice to store it in glass, too, after this experience. Thank you.