Thanks very much for all the suggestions. On reflection, my difficulties really started when i recently bought a set of Mavic Cosmic Elite UST wheels and i was wondering myself if it was a question of the combination rather than just the tyres. Comments here seem to confirm this. I fitted a spare (new) rigid Schwalbe Marathon tyre today and it went on easier than the folding Vittoria. I used more liberal amounts of talc than usual and i may well treat myself to a set of the VAR levers, which look quite natty.
Interestingly, having adjusted the front brakes i checked the rears and found they were binding considerably at the valve point. I thought at first that i had warped the wheel somehow, but actually the Continental GP that i fitted new a few months ago was bulging at that point; the bead was not seated properly. I think i would have noticed this amount of rubbing after i repaired a puncture a few weeks ago, so i guess it must have happened during use. Very recently i'm guessing as i've been finding myself more than usually knackered after a short ride!
Anyway, i've refitted it (at 180deg to before) taking particular care to seat it well near the valve and it seems OK for now. I guess we're entering maintenance and repairs territory, but i'd be interested in any observations.