Thanks for your views, Bessie123 and Love2Dance. Bessie, I think I get your bit about 'women who feel pressured to live a role' though I found it bit hard to understand/appreciate at first. I am someone who is generally pretty far from being pressured to live a role so maybe I just find it hard to see why I might find myself in that position post-baby. (I know you're not making a personal point here, seeing as we don't know each other in that way!) I take your point about seeing how it goes and the possibility of changing plans as I go on, but presumably my work will want to know my overall plans, even if they change, so thought I'd better at least have an idea...
L2D, I think some of the cons are partly what I have in mind when I wonder whether a full year's mat leave is a good idea - especially whether I will get to the point of killing for adult (or non-kid-related) company! I am not massively ambitious in myself but I really like my job, get a lot out of it, and (importantly) get paid well (DP's job pays reasonably but not as much). I am also in a fairly stable job (I know how lucky I am, believe me!).
Of course, as you say, career prospects will be affected by having a baby, but I have got a lot of experience under my belt and offer my workplace various skills that they will find it difficult to get elsewhere (and which they do need) - also, there are quite a few other mums who work 4 days a week, which is what I would expect to return to. But despite all those things that mean I'm very lucky, avoiding risk is also important to me so maybe not taking the whole year and using up all the contingency we have is a better idea.
Your last suggestion is sensible but not for me, I think - I'd rather know what I'm initially planning for and then change it as I go, rather than writing into the plan the fact it needs revising at an early stage, iyswim.