Loosen the backstay
Usually the backstay set set up hard, to keep the jib or forestay as straight as possible when under sail. These stays are probably under considerably more tension than is necessary to simply support the mast without movement, so I'd slack the backstay at least. I agree with capn jim ( agree with Bob Todd too, but I'm not lucky enuf to own a Nonesuch) : the shrouds & stays can impart a considerable loading on the hull structure and these loads should be relieived when the boat's on the hard. Actually, when not sailing, the backstay should be eased a bit at all times, then retensioned before setting sail. For winter I would slack everything a bit,but keep enough tension to ensure the mast cannot move.And 'lock' the turnbuckles so there's no way they can back themselves off even further.