I have found that fresh leaves tend to blow about and land on someone else’s allotment. If you can get your hands on builders bags they are good to stuff full of leaves and then by the Spring they will have rotted down enough to stay where you put them but are a better mulch if you leave them until next Autumn. Whatever, I find that leafmould helps so much to give the soil a good structure and I have successfully used it on both clay and sandy soil.