Thanks Steve, Drunkula, Foxbat et al.
This has been a very interesting discussion. I'm not aware that I've even got a issue with oxidation. My question was prompted by seeing Ascorvit on a homebrew suppliers website being plugged as an antioxidant in beer and I wondered if I was missing something. Maybe I am and I won't know until I've tried it. Indeed, I opened a very old bottle of very strong homebrew the other day and, although most of the citra character had disappeared, the beer was delicious- until the last mouthful when I detected a distinct "sherry" note. On the other hand, I've opened bottles of Duvel Tripple Hop- Citra (my wife's tipple of choice) over a year old and there's not all that much difference to a fresh one. Even the hops are still there!
So I'm going to set up an experiment along the lines Steve suggested above: a batch of strong, hoppy, blonde split three ways at bottling time: one with an appropriate amount of ascorbic acid as soon as I've determined what an appropriate amount is; one with 10ppm sulphur dioxide from sodium metabisuphite; and the last with no antioxidant- not even an oxygen scavenging cap.
I'll report back in about 3 months.
(Steve's comment about the present of copper: I use a coper wort chiller and while this puts insufficient copper in the wort to cause any toxicity to man or beast, there should be enough of a trace to catalyse the reaction he outlines above.)