Andy, as a diet controlled type 2, 4.5mmol/l is a remarkably non-diabetic blood sugar in my opinion!
You have negligable risk of hypoglycemia without blood sugar lowering medications, so there is absolutely no harm in running those sorts of numbers in your case.
Diabetics using insulin would tend to run bg's slightly higher to give themselves a little cushion between them and hypoglycemia, but that's not an issue for you.
All the best,
fergus