if you use pure ammonia, make sure that it's just ammonia and water, meaning no surfactants. if you shake it and it fizzes or bubbles then you
dont want it, no bubbles and you should be okay, but you can post the ingredients here and we can let you know if it's safe to use

oh, also you'll want to only add about 2-4ppm, which may involve adding a couple drops, waiting half an hour and testing the water (and hoping it's at 2-4), adding a few more drops, testing again, etc.
i'd check around at your local hardware stores, in the U.S. members have found it at Ace Hardware, but i'm not sure if you have that over in the UK.