@Rob Sedgwick Sorry for the inconvenience this must have caused!
I understand your concern about subscription getting suspended before reaching the spending limit. As you have mentioned it works when removed spending and you are being charged a few pounds. This seems to be unexpected behavior that needs to be investigated further to understand that root cause of the issue. This can be better handled as a support request for billing validation. Recommend you to raise an Azure billing support ticket using this link - https://learn.microsoft.com/en-us/azure/azure-portal/supportability/how-to-create-azure-support-request. The ticket enables you to work closely with the support engineers and get a quick resolution to your issue.
FYI: Azure Billing and Subscription Management support is included in the Basic Support Plan without any charge.
https://azure.microsoft.com/en-us/support/plans/
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