I had a long document with both header and footer but they have disappeared for some reason, whether I accidentally clicked something, I don’t know. I’ve looked at the “insert”/“header & footer” menu but it seems I’m not doing what I should to get the H & F back on all the pages. Any ideas please?
Maybe you activated the Seems that it’s not this, because you can’t choose Web mode (menu View).Print Preview in Web mode.
Whitespace could fiddle here also (menu View).
firstly : Ask/Guide - How to use the Ask site - The Document Foundation Wiki #More_details
then …
→ About Headers and Footers
→ Defining Different Headers and Footers
do they show up in Print Preview ? 
Sorry, where’s print preview?
Ah, found it, yes it shows a header & footer in print preview.
I managed to get a H & F back but… (you knew there’d be a “but”?) Both H & F are “shorter” than they were which has messed up my page numbers. I opened up another saved document and the H & F had gone from there too. When I got them back they were “shorter” too which messed up the page numbers on that document. Has there been some sort of update which has done this?
Now, if I click “Print Preview”, momentarily, the original H & F shows up before it goes to the preview. Has anyone got any idea what’s happened and how to revert to the way it was?
I saw there was an update, I tried that. Afterwards, the documents wouldn’t even open. So, I uninstalled LibreOffice Writer. Then did a new installation. Bingo, all documents went back to normal with both the H & F as they were before.