I figured it out. It is in fact the Bluetooth antenna, and it is indeed a separate module to the wi-fi assembly: https://www.aliexpress.com/item/New-Bluetooth-Antenna-Cable-For-MacBook-Pro-15-A1286-2011-Year-Version-MC721LL-A-Laptop-P/1691105968.html
Can anyone tell me what the little cable in the uppermost left hand side is? It's the fourth connection to the airport card, but is independent of the rest of the airport / wifi assembly. I damaged the very fragile connector and need to replace it, but I don't know what the component is, what it does or what to look for when sourcing a new one. Or, indeed, the method for replacing it. Any help very gratefully received.