Hi Margaret, welcome
And welcome to the wonderful world of not being told much useful by GPs and DNs.
The most important role of blood testing is in finding out what impact different foods have on you, so immediately before eating (not necessarily every meal) and 2 hours after starting. First thing in the morning can also be useful, especially if you experience significant liver dumping (dawn phenomenon).
What are you eating? If you are being put on Gliclazide I would guess you are following the DN instructions to reduce sugar but still eat carbs? Experience of many on here is that carbs are the problem and NHS dietary advice is rubbish.
Have a read of the first item linked in my sig below and ask any questions you want.