@Alistair Russell, Thanks for posting in Q&A.
From your description, we know that you want to know if a free account can use content search in MS Purview Compliance Portal and users with the licenses you listed can use content search in MS Purview Compliance Portal.
Based on my testing, I've found that as long as you assign Organization Manager and eDiscovery Manager permissions to your account, you can use the content search service in the MS Purview Compliance Portal even without an Intune License. Therefore, I think the free account can use Content search. You can try it.
Here are documents you can refer:
Get started with Content search | Microsoft Learn
Assign eDiscovery permissions in the Microsoft Purview compliance portal | Microsoft Learn
Hope above information can be helpful.
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