Where in the UK is area code 07761?

That's the beginning of a mobile number. Any UK number beginning with 07400 through 07999 is a mobile. That's +44 7400 through +44 7999 in international format.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)