Hello TimWelby,
what you are looking for exists. You can do this very easily by using the CloudDB component (in the appinventor´s „storage“ tab) that enables you to store text (strings and so on) under tags on the MIT or your own Server and, if you have pictures included, an extension that converts pictures to text (strings) since only they can be stored like this ([Free] Base 64 extension. File to/from Base 64), so you can download the contents if needed.
If you have further questions, feel free to ask me and the other community members 
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