I finished this a couple of days ago, but I don't get two of the clues. 21a: Navy yielding to e.g. Napoleon in Spanish town, not a real port (8). I assume that Napoleon is the PIG in the ersatz port, but don't see the yielding navy. 18d: Intermediate class with opening page, prompt to action (7). What's the intermediate class? Probably obvious to Brits...
Thanks, peterm, Chald, and AB. All clear now! At least I knew granger right off; there's a grange in every farm town here in backwoods Massachusetts. You can't imagine what words like cabinet and tonic mean here.