Before you apply for an allotment you should consider whether it's the right thing for you.  It's a proven fact that allotment gardening is good for you, both physically and for mental wellbeing, but working your own plot does take a degree of commitment that shouldn't be under estimated.  When a plot becomes available, it's often been left for some time.  The previous tenant may have given up the plot for various reasons, but mainly you will find it's because they were unable to keep it maintained to a suitable standard;  Often because thier circumstances have changed and they couldn't commitment the time, or because of ill health.  Plots don't come up often and when they do they are quickly snapped up!