I am very happy to report that, in desperation after none of the things I tried and that were suggested here improved the shifting performance of my 5spd hub, I lay the bike on its side and dribbled a good helping of that magic fluid called WD40 into the hub axle, and hey presto, for the first time in over a year the hub is working as it should. Is that magic or what! This is a variation of the 'lubrication' suggested above by the good Dan Burkhart.
I had initially used ordinary oil but that did not cure the problem.