Hi, how did you find out there were no vacancies? If it was from the Council, they could be a bit out of date. You could google for allotments in your area and if any of them have a website, make contact directly with them. Or maybe some sites have an 'Open Gate' policy at the weekend so you could visit and find out what their waiting-list is like.
If you get your name on a waiting-list, it's a good idea to keep in contact with the Secretary and ask from time to time where you are on the list.
Hope this helps,