Sandleford was the warren that was once home to Hazel, Fiver, Bigwig, and most of the other Watershippers. It was destroyed by a human construction crew, as forseen by Fiver. The only ones who heeded his warnings and left prior to the disaster were the aforementioned three along with Blackberry, Dandelion, Pipkin, Hawkbit and Silver. Holly followed soon after, the lone survivor of the actual destruction of the warren.
Hawkbit was not part of Sandleford, but Violet was among those who heeded Fiver's warning and left.
Silver was not part of Sandleford.