Royston, Hertfordshire, has only one Allotment site, owned and managed by Royston Town Council. It is a Statutory Allotment site which means that it cannot be sold for development without the consent of the Secretary of State.
If you are interested in obtaining a plot on the site you should contact the Council.
(Be aware that there is currently a long waiting list (about 6 years) so you should record your interest early and be prepared to be patient!)