[user104658]

Originally Posted by Kizzy

Wait till you've been there 26 weeks then ask to work flexibly

http://www.acas.org.uk/index.aspx?articleid=1616

I'd suggest being wary of official flexible working agreements as you can end up "locking yourself in" with your own requests, i.e. If your circumstances change in future you might find yourself unable to, ironically, be flexible because of your agreement.