The Kine saga, book 1: Kine / Marshworld
Alan Lloyd1982

A peaceful valley of woodland and marsh, little changed for centuries...
Until a ferocious invader appears - wild mink, savage and unstoppable.
It is left to a brave but tiny hunter, Kine the weasel, to fight back against Gru and her dark followers. So with Kine and his motley allies we spend one glorious, unforgettable summer in another world - a world where solitary creatures prowl secret paths and hedgerows, and whiskered legions gather for battle as autumn draws near...
Bewitching, cruel, fresh, poetic, sad - here is all the magic of the hidden coutryside.