OK, thankyou for all the responses. Just to be clear - I didnt' think i was entitled to anything, I just wondered if I did have any rights in this situation - which it would appear that I don't.
I did think - as has previously been the case, that you could automatically extend your career break for up to five years.
The situation in the now changed, and they are not giving anyone career breaks and are refusing extensions.
I would also say that I was not expecting a specific job to be held open for me, but also thought that I would have to apply internally for jobs, and would only start getting paid again if and when I found a job internally.
Ironically it now appears that as I am on my specific departments headcount they want to get rid of me so they can employ someone else in my place (or have me return).
I agree that working full-time wouldn't make anymore sense (I had it in my head for some reason that if I worked full time I could afford a nanny, but not part time - no idea why).
I know I am not entitled to have a job held open for me for 5 years, but I was lead to believe that they would, so I am , of course, disappointed that they won't. I can't afford to go back, just to keep the job for the future, we would be paying far more than I earn and we simply couldn't do that.
I am scared about the future now, a few years down the line, when the kids are at school, and I could cover the extra childcare as I feel that having had that many years out of the workforce, I will be virtually unemployable. HOWEVER, it is what it is, so we will just have to deal with it.