Hello MostlyLurking and all, I am an ex-drinker who has given up for good (seven years ago) and will never go back to it now except for the really occasional glass of wine with a meal (probably twice a year.)
"I have someone coming round tonight who will be"expecting" me to drink, what do I do without fessing up that I am trying to quit?"
Be absolutely clear so that it's obvious there's no room to persuade you, but don't get into a discussion (be casual and dismissive like it's not a big deal, which it isn't, really; there's plenty of people around these days who don't drink for one reason or another) and stick firmly to the 'never apologise, never explain' plan.
Use some brief phrase that's impossible to argue with: "It doesn't suit me any more, I find," is a good one, and then move on. Answer any follow-up questions like 'but don't you miss it?' with a smiling 'not really, no!' and change the subject with a swift (prepared) question 'Oh that reminds me! How IS DD getting on at school/your new job/your mother's impetigo' - etc.
Friends who used to know you as a drinker will move into accepting that you are NOT a drinker faster than you think, so long as you are obviously not wavering and there's therefore no point trying to persuade you. Be bland, vague, project 'it's no big deal' and move swiftly on.
Good luck to all.