These can be found either via MIDOS or on RADAR here: GP Backdoor numbers – Radar (radar-brisdoc.co.uk)
If searching on MIDOS you will need to enter the GP surgery details into both the initial two search bars. Entering the name of the surgery into the second bar can identify the postcode of the surgery if this is not known.
The shift manager can also support you with finding the backdoor number for local GP surgeries should you require this for any reason.