>>>probably should weigh 15-16 stone
Not unless you're about 7 foot tall! The healthy weight for a 6' tall man is between about 10 and 13 stone. If you're only 5' 6" tall, your healthy weight is between about 8 and 11 stone.
If you're perhaps 5'9" tall then you're not only overweight but on the borderline between 'obese' and 'very obese'. See here:
http://www.nhs.uk/Livewell/healthy-living/Pages/height-weight-chart.aspx
(You might need to press Ctrl and + together a few times to view that chart at an easily readable size. Press Ctrl and 0 together to get back to the default display).
The causes of oedema (which might well describe your condition) are listed here:
http://www.nhs.uk/conditions/oedema/Pages/Introduction.aspx
However it would seem to be likely that your weight is either the cause of whatever is afflicting you or making it far worse than otherwise.
Go back to your GP, both to ask him about the possible causes of your symptoms and to get some advice on weight loss. (This might be a good starting point:
http://www.nhs.uk/livewell/loseweight/Pages/Loseweighthome.aspx )