Thanks for replies everyone - only just realised it had got replies as forgot to check the last couple of days, sorry about that. Really helpful though. Will check about women's network at my institution.
I had sort of reached the same conclusion about only 1 or 2 children. I'm a bit sad about it as I come from a large family myself; but on the other hand I think there are real disadvantages to lots of children even aside from one's career, and in any case if I'd really wanted a lot I should probably have started sooner. Also my partner is really keen to have children, but only 1 or 2.
He is very supportive - obviously important - and his mother worked full-time, which makes him more confident I think than I am about the whole thing (my mother is very intelligent but quite dificult and controlling and honestly I think she and all of us would have been happier if she'd had more of a life outside the home). But I do worry that although he says all the right things his actual assumptions are pretty traditional. (For instance, I mentioned friends where she and her husband take turns to work part time while the children are small, and he was pretty appalled at the implication that maybe this is something he could consider. Like me he is ambitious about his career - also an academic.)
I have a good research record - mostly because I was lucky to have a research fellowship after my PhD - and I already have enough submissions for the REF, including a book which came out last year. So perhaps starting between now and the REF is a good time.
I would be interested to know how many of you have managed to negotiate any sort of part-time working arrangement and how that's worked out? My impression is that academia is good for families in the sense of being able to work a bit from home and be a bit flexible about hours and so on; but bad in that you are expected to work long hours at a whole range of time and especially because any sort of part time role seems to be functionally impossible. I don't know anyone working in that way, whereas my friends who are doctors, for instance, arrange this straightforwardly.
The other thing I am concerned about is that I have quite a high risk of hyperemesis in pregnancy - both my mother and elder sister suffered very badly, with repeated hospitalisations in all pregnancies, and I have several of the indicative risk factors (tendency to labyrinthitis; very prone to motion sickness, etc). So I'm worried about the possible impact of that upon work - I can easily imagine, for instance, being capable of working to some extent but being completely unable to commute.