Santander don't have individual sort codes for their branches; so it's no good trying to find the code for a specific branch. They use just a small selection of sort codes, such as 09-01-27 and 09-01-28.
So the only way that you can find your sort code (other than, say, phoning a friend or family member who has access to your debit card, cheque book or bank statements) is to contact Santander directly. Since the number is related to your own account, and not to a particular branch, they'll need you to answer some ID questions before they can provide you with the information you need.
Try a phone call to +44 1908 237 963. It's actually their number for lost or stolen debit cards but, even if they can't help you directly on that number, they might be able to point you in the right direction.