Saw my GP again today, and he's let me have metformin.
STAMPEDE dose docs said 850mg, initially once a day, and escalate to twice a day after a month if tolerated well.
My GP said they don't normally prescribe the 850mg dose any more. It's been replaced with a 500mg slow release dose which is less harmful to the stomach, and doesn't generate large fluctuations in level like the 850mg dose did, so that what I've got. Starting with one tablet per day, and I can escalate to two if I want to.
It's not allowed with contrast dyes for CT/MRI scans (they're both quite a load on the kidneys), and having had one of those yesterday, I will wait a day or two more before starting it.
What side effects did you have? GP mentioned it can impact bowel movements, and some people come off it due to that.