Need to send a doc in odf - how do I save with the extension in ODF?

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I don't think you have correctly understood what Inland Revenue want -- but you should supply the text (or a screenshot) of the instructions requiring an ODF extension.

The previous answer provides some very helpful links -- and they make clear that what is needed is a document in ODF format, not a document with an ODF extension. Their own "ODF" document types includes the *.ods spreadsheet in ODF format, for example:

And note that this is the expected file extension for spreadsheets (not ODF):

I take it your document is produced in LO's Writer. If so, the “Open Document Format's” (odf's) extension should be .odt. So select “File” > “Save As”, open the “Save as type” list and choose “ODF Text Document (.odt) (*.odt)”.

Appendix B of LO 5.2's “Getting Started” user guide introduces open document format and explains the extensions used in the other LO applications. You can download the guide free from the Document Foundation's documentation pages (https://documentation.libreoffice.org...).

I take it that your reference to the Inland Revenue means you are based in the UK, so you might also like to look at the UK's official guidance for Civil Servants (it is quite readable!). You can download it free from the Cabinet Office website (https://www.gov.uk/government/collect...). And there is a lot more Cabinet Office advice as well from this site, the guidance on meeting user needs (https://www.gov.uk/government/publica...) might be useful).