I went back to work last year after two year break and career switch. It felt like a real victory.
After a few days I realized I was the only mum at Canary Wharf (apart from the very few who had remained at work on throughout and now had the tenure to work flexibly/part-time.)
My boss (the archetypal unmarried career woman) hated me for needing to leave a 5.05pm and pretty soon had forced me out.
I suspect that all City firms or similar are going to be the same in terms of demographics/attitudes, but if anyone knows an exceptional firm that offers genuine work-life balance to new entrants (with great credentials, natch), I'd be glad to hear its name!