Try the place just the other side of Hertford Football Club at the end of West Street. Lots of horses there, best manure going. Think they're running a surplus so would probably be happy to accept a donation to a cause.
Thanks Mack. I also found another place - Petasfield Stables in Mangrove Road. Lovely friendly place with stacks of free rotted manure, all they ask for is a donation to a horse charity if you want to.