Hi @RUSSELL SHEEHAN
According to the article: "My OneDrive says it's full", I suggest you check your Microsoft account mailbox: https://outlook.live.com/mail/0/options/general/storage.
If you are sure that the email data can be deleted, you can try clicking "Empty" for some folders.
I also suggest you go to OneDrive online, check whether you can clean up some files or folders to make more space.
If you have issues related to OneDrive on iPhone, which is not supported on Q&A forum currently, it's best post a new thread in Microsoft Answers community: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_drive-msoffice_OfH-mso_OPiOS?sort=LastReplyDate&dir=Desc&tab=All&status=all&mod=&modAge=&advFil=&postedAfter=&postedBefore=&threadType=all&isFilterExpanded=true&page=1
Thanks for your understanding.
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