Let's start with a picture:

This is pretty much the same as https://answers.microsoft.com/en-us/msoffice/forum/msoffice\_officeinsider-mso\_mac/the-ability-to-select-the-office-insider-program/812a6957-7bc5-48ed-b0a9-413d78c9ffd2
However, I have additional information that may help:
OS Version: MacOS 10.12.6 (Sierra)
Current Office version: Version 16.7 (171029)
Licence: Office 365 Subscription
My Mac is not wholly administred by an IT group - it is my personal MacBook Pro, but I one of customers has loaded some stuff to force me to encrypt my drive and a few other things (had to agree before I could get access to confidential documents), but I
still have administrator access (via su if all else fails)
My suspicion is that my customer has done something to disable access to the Office Insider updates.
Does anyone have any clues as to what they may have done and how to undo it?
Other info that may or may not be relevant:
I also have a Mac Mini, on which I am happily running the Office Insider Fast versions, currently 16.10 (180109). The only difference is that the Mac Mini is running OS Version 10.13.1 (High Sierra)
I have been through every privacy setting on both my MBP and Mac Mini to ensure they match.
TIA
RedNectar
aka Chris Welsh