Apparently not all filters are the same.
I went for a higher grade one from Gaps Water than the one I usually buy. I assumed I was wasting my money, but the filter that arrived was significantly different from the ones I'd bought before. I'm not saying better (though Gaps were) just very, very different.
After shagging a membrane through what I suspect was inadequate filtering, I no longer buy solely on price. Mind, I have now gone to two particulate and three carbon, so I could probably use toilet rolls and get away with it.
Vin