Tiger pee sourced from the zoo nearby did the trick, according to the other allotment plot holders. Never got around to trying it myself, and I've moved away since then. Scares the bejesus out of cats does tiger pee, apparently. Also tiger poo. Also tiger poo. They knew a guy.
You can cut the bottoms off 2ltr drink bottles to make small protective greenhouses for saplings. Leave the lids off. Or make fences using bamboo and netting, cats will be put off even if they're not high (fences, not the cats [unless you plant catnip]).
Lay down semi-permeable membrane and cut out circles for the saplings, minimising cat digging area. Once one cat marks their terrirory using pee and poo, others will follow, leading to conflict.
Also, if your garden has ever had rats or mice pee over it, cats will never stay away because it becomes valuable real estate, leading to cat pee, poo and conflicts as they all claim it as their own; horrible injuries and fights to the death.