It's been a while since my last post – this being because the phenomenon was very haphazard and infrequent so I needed some considerable time to get a measure of it.
I had thought that the “suspend” function was at the heart of the problem as opening the lid (set to suspend) appeared to be the most frequent precursor of the wireless failure. However, if it is
a contributing factor, then it's not the only one as there seems to be no particular pattern.
Just to compound matters, the wireless-to-internet non-connection has diminished of late, to be replaced with other problems associated with LibreOffice. Often, when Alt-tabbing between applications, returning to, say, a document shows missing icons in the menu bar or, more frequently, overlapping, blank icon tabs from a previous application. Sometimes, the machine boots with some of the top panel (classic view) icons missing. The icon positions are present, as I can click on the blank spaces and they appear, even if only briefly, and work OK.
Overall, I am thinking that a complete re-install might be in order, but the effects are probably not frequent enough now to make it worth while. I have just loaded Ubuntu 16.04 onto a mate's brand new Lenovo B50-10 and I have seen a few minor glitches...........perhaps it's a Lenovo thing. Neither the B50-10 nor my T500 appear in the Ubuntu/Lenovo tested list https://certification.ubuntu.com/certification/make/Lenovo/
(although the T510 does) but I'm clutching at straws here.
The verdict: I have no idea. Sorry. But thanks for your support.