thanks all. I may have given the wrong impression re what dh said. and he is absolutely not sexist. in actual fact he's quite a trail blazer in his v. male oriented industry. Several years ago he negotiated a 4-day a week contract so he could spend more time at home. it's virtually unheard of for men in his line of work to do this (he was featured in a newspaper article about dads who work part-time). he then resigned to go freelance and now works 1-2 days a week. he is currently negotiating a new permanent contract for 2-3 days a week, which will give him loads of time at home but a more secure income. he does more parenting than me, is better at it, enjoys it more and prioritises it way over work. if he could he would give up work completely but unfortunately we can't afford that.
when I said he did once say he'd miss me if I worked away too much, what I should have said really is "if we both worked away too much".
however, I will admit, that there is a sense in which gender politics does, unfortunately, creep into the scenario - in that his work is extremely well paid and mine is not. (both similarly qualified, but he went for maximum pay for least effort, I went for a vocation) so, as we plan our life, there is an assumption we can't (financially) get round - that he will work and that some of that will be in London. that's our starting point, if you see what I mean - hence I said "if I work away", because currently he does and I don't. it's not sexism at a personal level, but perhaps has somthing to do with gender at a societal level.
the conversation where dh said he'd miss me if I worked away "too much" was also about a different scenario to that in my op - one where I worked away more than him - 3-4 days a week.
the other complication in working out what's "fair" is that I would be doing something I love and am committed to, whilst earning very little - in effect taking less than my fair share of family (ie childcare and financial) responsibility. if the roles were reversed and a dad wanted to pursue his dreams in this way I know what the mn response would be.
I realise much this info wasn't in my op - which was much more about logistics. Iadd it now because some of you read stuff that wasn;t there - esp xenia postulating that dh thinks he has a little wife, and ladylapsang thinking dh was trying to stop me. I've never had anything but the best support from dh. including when I travelled to a war zone when dd2 was 9 months and dd1 2y9m. and including when I took a year's sabbatical during which I worked 4 days a week and earned zero.