We have an old Astracast tap which is fitted to a hole in the back of the double sink. There is a single spout for both hot and cold. Can I assume that a similar-looking tap from my local plumbing shop will fit this hole, or do they come in different sizes? I shall get an expert to fit it, of course, but don't want to buy the wrong size. If I need to measure without removing the tap, can I just take the diameter where it enters the hole as a rough guide?