ellabella2 - with you completely.
Normally I'm a 30H/HH (dependant on the style of the bra). However, this pregnancy I've shot up (again) ad am currently bulging out of my 32J which I got at bravissimo. I'm waiting on a 32K nursing bra from John Lewis.
I get fed up with my boobs at the best of times, but pregnancy brings out the worst in me for boob-moaning. If it's any comfort though, OP, they haven't grown at all since the middle of the second trimester or so. I think the point at which your boobs come in does vary from person to person, but once they've done their growing they don't tend to have a second wind and put out again later on.
Mine did go down again post pregnancy (couldn't breastfeed because of the medication i'm on) but as I had never been 'properly' measured before getting pregnant, I couldn't tell you how close they were to original size. I think they were a little bigger though.
Oh, and another one here saying avoid M&S like the proverbial plague. Mothercare, too, don't do fittings particularly well. I always sagged before getting pregnant because I didn't know about shops like bravissimo, so it was ironic that my boobs got a lot better shape aftr my DS because of the better bras I was wearing. The trouble is, tape-measuring a woman for a bra while she's already ina bra is only going to be as accurate as the bra she's got on, especially if she has large boobs. Plus, IME they don't check to see how it sits once they have measured. They just look at you as if to say 'well that's what the tape measure says, you're just making a fuss' if you say it's uncomfortable.