What everyone has said so far is good. Also, there are a couple of different forms of metformin, plus of course, there are other diabetes drugs altogether, so maybe a different med may suit you better. Straight Metformin tends to be an opening gambit by the NHS to get your sugar under control.
Also, I seem to bang on about this every time I post here, but it really is worth testing yourself regularly. Metformin is prescribed for a specific purpose, and you can measure for yourself just how effective it is. There's no magic going on. Medical professionals will go on about your hba-1c value, but if you take regular glocometer readings (which are basically spot values) then you get a picture which is just as good, and in the privacy of your own bathroom.