However hard it might be to get in touch with your GP, you really ought to find a way to get to grips with the cause of your problem, rather than just trying to deal with the swelling itself.
To the best of my knowledge anyway, there are no diuretic drugs available without prescription. That only leaves 'natural' products, such as those sold by Holland & Barrett but I note that both of the products sold by that store state that they shouldn't be taken by people with diabetes (which, if I recall correctly, includes you).