Welcome to the forum
Many people find that blood glucose can increase when people have any sort of infection or feel unwell.
The metformin will probably have no immediate effect as that just works away in the background however taking extra gliclazide may help. Gliclazide encourages the pancreas to produce more insulin to help metabolise the carbs you are eating.
If your levels are remaining high then making sure you don't eat too much carbohydrate and staying well hydrated should help it come down.
When did you get the 15mmol/l in relation to when you ate.
What sort of dietary approach do you follow as your levels seem quite high normall