I think the first thing we should try is to avoid dehydration; this can cause nasty tastes. Two and a half litres of water a day seems a lot, five pints in English! But that is the recommended water intake for a healthy 11 stone person, so if we check on a pro rata basis, just about three pints (1.8 ltr) would suit a person of eight stones. Do your gums bleed when you brush your teeth? That could leave a salty taste. That irritating, embarrassing thing, constipation is said to be one of the most common causes and you would have to attend to that.
NHS choices lists things like medication, gum disease poor diet.
Gastric reflux is my least favourite and has been my unwelcome companion for years, watch out for ranitidine, it can contribute to constipation and a catch 22 situation. That's where my research has led me so far. Toothpaste, mouthwash and plenty of fluids seem to be the order of the day. I am not a doctor so the observations I have made are based on my own experiences.