"Local GP not much help, anyone got any suggestions?"
I would definitely fire the GP and find an Endocrinologist, a diabetes specialist, for complete testing. Low glucose [blood sugar] can be dangerous and life threatening. Too low a person will go into diabetic coma. Some recover, some don't.
As far has his weight, beer can raise glucose levels as does simple carbohydrates. The calories in beer will make one fat, get a "beer belly". Like asok mentioned, find a dietician/nutritionist. Not familiar with the UK but in the states they are usually found at hospitals or possibly the endo can refer your partner to one.
Go see a registered dietician who will probably suggest avoiding carbohydrates. Mine told me only 60 to 75 per meal. I have lost 60 lbs since diagnosis.Got to go now, time for my 10oz New York strip. No carbs.