I've just registered with shiftshack.com
It's a service that charges 99p per and for that you get an online calendar which you fill in to show employers where and when you're available to work, you also fill in details about what kind of work you want etc. The website then matches you to employers who need people to work on a part time, flexible or hourly basis.
I'm not going to build a career like this but at least I can manage my time and earn money around the kids. I would really recommend it.