As I mentioned before, I use vinegar just because it is handy and I don't notice the smell. We buy it by the gallons at Costco, cheap. My sleep tech also recommended it for use on the hose and mask, and said it would also act as a disinfectant. However if you smell it, at least for cleaning the reservoir, you could try citric acid powder. It is the same acid in lemon juice but probably less expensive without the mess of using lemon. Citric acid is what gives some candies the sweet and sour bite, and is used in some recipes for cooking where you want the sour without the vinegar taste/odour. It is used in home beer making, and you can buy it in reasonable quantities at a beer and wine making supply store. I keep a bucket of it on hand for cleaning the coffee maker, the dishwasher, and our front load washing machine.
That said, I think if I was in the UK and couldn't buy demineralized or distilled water I would buy one of those ZeroWater gizmos. No, they are not the cheapest way to get demin water, but if you can't buy it, then they may be reasonable. I think the economics for filter replacements are in the order of $1.25 per gallon of water. That is not much more than you would pay for it in a store, if you could buy it... Amazon in the UK has lots of them for sale.
ZeroWater at Amazon