In my experience, you absolutely have to show your face. People just ignore letters, phone calls and emails.
Turn up to the site ~ there's usually somebody there on a Saturday before lunch ~ and get chatting. Find out which ones are empty, meet the Rep, get your name down.
It may not be right, but Reps will give a plot to someone they like, who seems keen, rather than someone "on the waiting list" that they don't know from Adam.

(I am thinking of giving up half of my lotty ~ and I want it to go to somebody who will turn up & keep the weeds down, be chatty but not a pest, and who won't use sprays. If I don't get someone like that, then I won't give up my plot at all.)

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~ What do I think of Western civilisation? I think it would be a very good idea ~ Gandhi