How long have you been taking Metformin and how far apart were the HbA1c readings?
500mg Metformin per day is a minimum dose, so I presume that is what you were already taking and the doctor wants to increase it as your HbA1c is rising.
And as it would be unusual to check an HbA1c level six weeks apart, I also presume that the weight loss is a new symptom and not coinciding to a medication change.
So to me I would think it unlikely the weight loss is related to Metformin. When it does cause weight loss it is usually a slower and gradual process.
Has there been any change to your diet? A loss of appetite? As weight loss can be caused by many different factors, I would speak to your G.P. about this specifically so it can be investigated in its own right. It may have nothing to do with diabetes, but a sign of something else that needs treating. And unintentionally losing 5% of your body weight over six months is medically significant. So even if over 8½ stone, a trend of 6 lbs in six weeks is high enough to seek help.
If Metformin is otherwise helping I would recommend increasing that over Gliclazide. The latter will create the risk of hypos and so will need constantly monitoring and awareness of those so the "side-effects" are more significant. Particularly as you do not have a problem tolerating Metformin.
As for the suggestions above questioning your type, have you been experiencing any other symptoms the last six weeks? Particularly more frequent urination, fatigue, blurry vision, or increased hunger. Those in addition to weight loss would suggest you are not producing enough insulin.
But you really need to discuss the weight loss with your G.P. so it can be explained.
