##UPDATE##
So! For no apparent reason ( weather, warmth, cold, hayfever etc. ) Archie started to have his air-scratching episodes once more. No where near the worst, but enough for me to get more tablets from the vet. I gave him 1/2 x 16mg ( 8mg) yesterday tea-time and another one this morning and we have seen an improvement. He has barely scratched at all today, so it looks like these Apoquel tablets work from minute 1. He has possibly slept a little bit more, but he is 14! I will give him 8mg for next couple of days now and se how he is at the weekend.
He is still a Happy Chappie
Thanks for your interest.