Our London colleagues agreed with our improvements and arguments, and suggested ways to improve the cover letter. So now we'll see if the editor also buys our arguments.
Too bad I'm just one of the middle authors. I feel like I've done a lot of work on it, both actual experiments (the time course I did in December) but the senior author is, rightly, the London investigator whose grant supported most of the experiments, and the first author is, rightly, the person in his lab who did those experiments. My post-doc is sharing official-first-author credit with that person, largely I think because she's taken the initiative of putting the paper together and doing most of the writing.