Might be pricier than elsewhere but Halfords.

Bosch. Does your car have the Ford smart charging system?
Go to a motorfactors and rock out a Bosch!
Worth taking into account Varta and Bosch are the same battery with a different label can normally be purchased for 10-15% less.
I started getting the low battery warning message on the dash and didn't drive the S-Max anywhere for 3 or 4 days but the battery was dead after that - could unlock/lock the car and use the stereo but it wouldn't start. It's possible that as your car is driven a lot more that you haven't noticed the problem until now.That's just what I hoped for. I'd done some browsing on ECPs and found a Bosch. I'll get one tomorrow and my 2 hours run down the motorway should at least last me to get to work tomorrow.
Looking at this one:
http://www.eurocarparts.com/ecp/c/F...b83fbabdfb375c5c92b807b6aa74a749cc0ab1&000020
Have you tried calling your local motor factors?