So, after making my first 'batch' of mag water last night I tried a small glass before bed.
Within about 10 minutes I had the most severe reflux symptoms I've had for months. Lump in throat, slight wheeze in chest, etc. but I don't want to just assume.
FWIW, I actually do have IBS and reflux. Having said that, knowing what MoM is actually used for means this makes little sense that it would actually cause a symptom it's meant to help. Many reflux treatments also contain magnesium so, again, makes little sense really.
I mixed 45ml with a litre of soda water, shook, left and shook again. Then when settled I mixed with 4 litres of normal water and drank a tiny glass of it.