I have used KilRock "Liquid House Power" "Household Ammonia", they sell it in Robert Dyas but I think that is very much a London only group of hardware stores but I checked with the manufacturers first that it was a pure 10% Ammonia solution. I did the same thing and went to Boots first, I am sure 10 years ago you would have got it there but they seem to have started angling themselves more up market.
Tel for the co is 01494793900 if you want to find who they sell it to.
Woman I spoke to was surprised at what I was using it for!!
StatMan