Parish Online can be used to manage your allotment plots. There are two pre-loaded layers for you to use:
Allotment Boundaries. This is a map of all the allotment sites. It has one area representing each allotment site. Within each area is information about the site as a whole.
Allotment Plots. This is a map of your individual tenanted plots. Each one can include whether it's tenanted and the details of the tenant.