First of all, I'd like to say a big heartfelt thank you to everyone for all your comments and support. You're all absolute stars.
Now that I've stopped wailing and tearing my hair out and have managed to drag myself off the funeral pyre that is Chapters 1-3, I'm getting stuck into the rewriting. And I'm actually quite enjoying it.
The editors were absolutely right, and most of their comments reflected what I'd already guessed. Basically my first three chapters fell into the trap of relying on sexual attraction cliches and didn't really identify the core conflicts of the characters. I'd also got too caught up in sparky banter and had kind of forgotten about motivations and consistent behaviour (which I think may well be the cause of the problems I've been having with a saggy middle). Oops.
The story now starts from a different point, the minor cocaine-themed subplot (which doesn't really fit in with 'fun sexy escapist read') is going to be replaced with anorexia-plus-a-fondness-for-appetite-suppressants-themed subplot, and I'm hoping the middle will turn out to be nice and toned.
Fingers crossed I've got it more right this time...