I mentioned on a thread a couple of weeks ago that I was going to visit the following place (AJ Wyatt) which is on the list of Megazorb suppliers. Have just come back and they stock it for £5.75 a bag and have a lot of other rabbit supplies (most brands of dry feed etc) in the farm shop. May be worth a visit if you are in the Bath/Bristol area.
http://www.qype.co.uk/place/preview/uk-2625538-a-j-wyatt-animal-feeds-supplies-bristol
http://www.qype.co.uk/place/preview/uk-2625538-a-j-wyatt-animal-feeds-supplies-bristol