Well! Solved at last after literally hours of expert Apple support phone calls. Very grateful to Apple support, absolutely outstanding, but I finally twigged myself what the answer was.
Reverting to iTunes 11 had prevented me downloading apps or updates to ones I had already purchased. System updates downloaded fine but I was unable to download 3rd party application updates and even Apple's own software purchases such as Pages, Keynote and Numbers.
Finally, I installed iTunes 12 - problem gone!
Not sure I can live with iTunes 12, but I could still go back to iTunes 11 again as in my earlier instructions.