Hello Martyn,
You can pull the Service IDs for all services via GET
https://deskapi.gotoassist.com/v1/services.json
or GET
https://deskapi.gotoassist.com/v1/services.xml When you GET records (incidents, changes, etc), the service id for the service they each belong to will be given to you as well in the response.
I hope that helps, let me know if I can answer anything else for you.