Question Author
Thanks for your answer. Unfortunately we cannot have separate taps as we have a new sink that caters for just one hole. Since posting, it has transpired that I need mixer tap that's suitable for low pressure system.
Re the issue of the smaller holes, I thought the smaller hole was suitable for combination boiler systems (of which mine isn't). the fitter said it was smaller because it was an Italian design!
An independent plumber I spoke to over the phone said we required a "bi-flow" what ever that means. Do you know?