Martyn – sorry I did not pick this up when you first posted, but I was in Seoul and run off my feet. I agree with the comment that it is mainly a humidity problem, and I find it worse on overnight flights, maybe because I am asleep with my mouth open. I also agree with LP that a simple water spray is a good prophylactic – I was interested to see that Korean distributed these in the business cabin after the meal and before dimming the lights. Even so, after the flight out I had a bit of a sore throat and a very congested nose.
I also agree with WM2010 that viruses are a problem. All argues for better humidified and better filtered air in the cabin.
Alcohol is good for many things – but mainly because of the effect on mood. I am in T4 this morning taking the central European preventive measure against odd infections creeping in … a tiny vodka before earing or drinking anything else.