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My "house" internet is the mobile dongle on the 3 network and, since I don't (yet) have 4G on that, there's not a lot of speed difference between it and the phone tethering.
The spreadsheets involved are my own creations and there are no hyperlinks within them.
So that leaves the online help possibility, and that's why I mentioned that Word has no problems because I reckoned that something like help would be common to the Office suite - have just tried Powerpoint and OneNote and there’s no problem with those either. Also I have checked that there are no active addins so that seems to rule them out.
Oh well, it doesn’t matter since I only occasionally use tethering.