Ok: the next day the Young's yeast in sugar water had produced a patch of froth on the surface. Most (all?) of the yeast granules were lying on the bottom.
I bought a fresh tub of Wilko's wine yeast and repeated the sugar/water/yeast test. Again, no reaction until next day, when there was an identical patch of froth on the surface. Again, most/all of the granules were lying on the bottom.
Not only the reactions but the Young's/Wilko's granules themselves look 100% identical. By end of day 2 the froth had died away and there is no visible sign of activity. So either:
1) They're both stale
2) Wine yeast is always subdued in its reaction to sugared water
3) I didn't use warm enough water (just above room temp)
If anyone can confirm which of the above is correct I - and I'm sure some others with similar experiences - will be eternally grateful. Otherwise I'll continue to use both tubs and hope for the best.
Chippy/lads, thanks for your indulgence and advice.