Hi all, thanks again to everyone for your help. I think I have solved this satisfactorily, and in my case I was probably worrying too much, but at the same time I think I'd advise full removal in cases of doubt for the reasons discussed above.
I wasn't happy with the amount of Office leftovers I was finding, partly because of the activation failure first time round, but at the same time I was nervous about the manual uninstall procedures I was reading (partly because they were inconsistent), and of using Revo; which was silly really, as I've edited registries quite happily in the past, and people seem to have a lot of confidence in Revo and don't have problems with it.
When I ran Revo (but baled out without actually cleaning) it found a lot of Office related registry keys, but all Click to Run, rather than Office itself. The Office set up I had was click to run, I hadn't actually installed or activated the 365 trial, and after having uninstalled my previous installation attempt and cleaned up file and folder leftovers I couldn't see anything directly Office left.
The retailer's download / install instructions didn't say anything about cleaning to the extent I was considering, so in the end I decided just to go for it, ran the installer and all went OK; I hope I'm not speaking too soon, but I now have an activated and functional Office perpetual licence installation, no sign of duplication, 365 or activation issues, no 365 / installer stub in Start, just Word / Excel etc. as it should be.
Because there's actually nothing "installed", that entry in Apps is just an installation "stub", a bit of sleight of hand by Microsoft to make users think it came with Windows, click on it, then unsuspectingly have to follow the steps to purchase the product from Microsoft ... and unknowingly accept the default setting for auto renewal and get trapped for life !! I call it predatory marketing.
EDIT
And as I have seen countless times, the dangers of leaving it (the app stub) there and just installing a full version "over the top of it" is that users inadvertently mistake the icon in the Start menu for the subsequent fully installed version, click on it and it proceeds to (re)install Office, thus stuffing up the legitimate previous installed version
... and my retailer's instructions, and Microsoft, specifically warn about this for those reasons
Also, installing ANY version of MS Office over the top of any already installed version of MS Office is fraught with dangers and often ends in tears. Each version of MS Office has its own in sub/folder in Program Files (Microsoft Office 13/14/15/16 ... etc) so a nerw installation doesn't replace/overwrite the old/existing version. Consequently, Microsoft's "control" of the default program for Office components (when more than one version exists) is chaotic, to say the least.
Microsoft seem to suggest the click to run or MSI installers should remove previous versions, but in reality that doesn't seem to be the case; they seem to either ignore previous versions, or fall over if there's interference between the two. I've probably spent a disproportionate amount of time worrying about this and trying to get to a definitive answer, but I think I'd recommend full removal if there is any doubt (and you're not dual installing?)
Thanks again for everyone's thoughts, they've been very helpful.
Why They Conflict
, then there is no choice but to turn to Ratiborus. 
