Thanks Robbity. That was a fascinating article and made sense of so many musings I had but never got around to investigating. So my question should really be would this one meal interfere significantly with my fat adaptation such that I have to go through 'keto flu ' again. Presumably not, unless my fat adaptation isn't complete? That still leaves me wondering what fat adaptation is. I had, simplistically, assumed it was the process of cells producing more enzymes for the metabolism of fats rather than carbs and (pessimistically I now see) assumed that it could be reversed by eating carbs again, even just for a day. I guess it might take a bit more than that to reverse things - like eating carbs over Xmas!
Now I know that I will have to avoid the temptation of occasional slips. Previously I was discouraged by the fear of re-adapting. Sometimes ignorance is bliss....