Hi Geoff Allsopp,
Please first check this thread (https://learn.microsoft.com/en-us/answers/questions/1629031/cant-find-subscription-id) to check if you have the proper permission and you are in the right tenant.
Then follow this thread (https://learn.microsoft.com/en-us/answers/questions/1481572/help-find-my-subscription-id) to get your subscription id.
If the above response helps answer your question, remember to "Accept Answer" so that others in the community facing similar issues can easily find the solution. Your contribution is highly appreciated.
Regards,
Luis