OMG .. you know what? My laptop lost connection too this morning - the WiFi was connected but there was no connectivity. Then, an hour later, BOTH the laptop AND the PC came back online. Maybe, in spite of the laptop working, this was a fault with the building's WiFi and they've just fixed it (there are no office staff here at weekends, just security). If that is the case I am REALLY sorry to have caused you kind folks to rather waste your time on a wild goose chase like this - although all the information is very useful. The fact that the laptop was fine lead me to believe that the fault was with the PC, not the WiFi system itself. I also did compare the ipconfig results, PC and laptop. The laptop was fine, with all components, DNS etc., as they should be. The PC was, well, as posted.
Well, early days: I have only tried connecting the PC to the internet once, so will report back again later to say whether all is OK. I am going top do a repair ('upgrade') install on the PC anyway, as the boot time is still way too long and red error messages, Event 100, are showing in Event Manager - doing a repair fixed this problem before, (odd though that restoring an image of the C drive made when booting was fine does not fix the problem???).
Will post again later today. Thanks again.
Martin