Can you not get a capacity check anywhere?
I thought Halfords and such offered a test for free.
On my car it was done with a printout of the result during the service.
The test showed the battery was only holding 63% or its original current in summer and so needed replacement.
Halfords seem to offer the battery test as a free "car check":
http://www.halfords.com/webapp/wcs/stores/servlet/ACArticleDisplay?storeId=10001&articleId=788003&categoryId=245348
John