If you can seriously consider going back to a shop with something that possibly cost less than a quid, then you have too much time on your hands.
If you are still alive the next time you are going back to the shop then take it with you.
Or not enough money, al. I once took a tin of rice pudding back to the shop because there was no rice in it, just the milky liquid. Yes, I was laughed at. I'm glad it was just across the road.
Anyway, I think Bazile is concerned about the health risk, not the money.
Dunnital in this thread you are the only one who has mentioned 'compo'... sorry I can't add to that.
Good advice from rc though ....
Baz the fact you haven't posted in almost a day may suggest to some that you have met your maker .. my guess is that you are right as rain and still gazing at the empty bottle and wondering what to do ?
Google 'Indian tonic water' and you will find that it's ingredients are not suitable to sustain the life of invertebrates .. Indian Tonic Water was initialy used as a medicine to ward off Malaria. The fact that it contains Citric Acid which is a well known disinfectant would alarm me more than a stain on the inside of the bottle !
The fact that everyone is selling it, would suggest that you are more than likely see this xmas in .. hopefully with the rest of us ...
Speaking as someone who has just spent 8 hours in A&E
and seen how fragile life can be .... add plenty of gin to the tonic, it will kill off the critters .. Cheers !