I think the sacking with creosote is for codling moths on apples. Plum moths need a pheromone trap in spring to control them.

Usually rotting plums is a sign of poor air circulation through and around the fruit. Rainy days, as the fruit gets to the ripening stage, don't help either. Perhaps a little judicial thinning earlier in the year might help. Some old trees are prone to the rot problem, often because of the huge amount of fruit. One plum rots, and because it is touching others, the rot can spread easily.
Phil J