Thanks all, for the comments. Ashbeck is still cheaper than either Zerowater or Osmio. (No-one has mentioned Skuma.) As for environmental impact, both the alternatives use disposable filters so do produce some waste and Osmio requires electricity to work and produces waste water. On balance I’m no more bothered about the 5L plastic bottles, which get recycled, than I am about the others.
When/if I change to a dual boiler machine I’ll revisit the question. For now, I think I’ll stick with the devil I know.