The National Allotment Society have a Growing Advice section which includes how to plan an allotment, month by month suggestions, information about different types of fruit and vegetables that can be grown and tips on different gardening methods. Their website also has a resources and downloads section with information on a range of useful subjects for the beginner and the seasoned allotment gardener.
The Allotment & Gardens website is run by John Harrison. It is packed full of advice and tips from his 40 plus years experience. There is also a forum where expert members can share answers to help with growing and poultry keeping questions.
The Royal Horticultural Society have an allotments section on their website with advice on how to get started and making the most of your plot.
C&DAHA members will also share advice on this website. Check under the Growing Advice menu for more links.