As an alternative, look out for football referee shorts.
Particularly with cheaper shorts - I'm looking at you, Karrimor - the rear zip is often stiff and poor quality, and often has a needless flap of fabric over it which gets easily caught.
Ref shorts on the other hand have tend to have deep side pockets (some zip, some not) as well as a secure pocket at the rear.
I got some with velcro rear pockets from the Adidas January sale and they're superb for gels, phone etc. Nike and Hummel also do very good ref shorts with a zipped rear.
Pretty much all of them RRP under £20. The Hummels can be had for just over a tenner.
Edited: 22/03/2013 at 11:41