I read that post about advchk.dll, found mine was as suspected o.o.d.
Simply uninstalled Spybot S&D and reinstalled from the "safer-networking" site this time rather than Download.com.
Whether that made a difference is just guessing. but I updated immediately and ran it. Found a couple of minor nasties like Double Click.
Ran a second time and came up with a no-nonsense big green tick.
Put simply, this sort of problem often only needs a careful re-installation from a trusted source.