Hey Cosy!
I can't advise on taking the Metformin, but if you have taken double your normal dose of gliclazide, you could hypo later this evening as it works by making your pancreas produce more insulin. My feeling would be don't take the Metformin as it makes your cells more receptive to insulin and the Glic will be making you produce more insulin, so it would be best to be less receptive to it under the circumstances I would think. But I'm not Type2, so hopefully someone on your meds will advise.
Do you have a testing kit? If so, test fairly frequently and set your alarm to wake you up to test at least once overnight. Personally, I'd give NHSDirect a call and see what they advise.
Let us know how you get on
Smidge