Hi,
Sorry to hear about your health problems. They are familiar to most of us here so you're in good company!
It would be useful to know which tests you have had done by the GI doctors you have seen. Have all of these been gastroenterologists in the NHS or private practice?
Mercury is a huge problem since it compromises the so many metabolic reactions in the body and seriously impairs immunity - allowing pathogens to take hold. It may surprise you to find out that the gut holds the majority of the whole body's immune system so an impaired immune system invariably leads to overgrowth of undesirable bacteria, yeast and other pathogens that would normally be kept in check.
I recommend you find a doctor through the
British Society for Environmental Medicine
I also recommend you read about the
Comprehensive Digestive Stool Analysis (CDSA) and
Intestinal Permeability (IP) tests which will give you a lot of useful information a gastroenterologist will not (assuming you haven't had these tests already).