Is anyone aware of allotments being run as a cooperative, either a single site, or a group of sites?
In what sense do you mean Robert? An Industrial and Provident Society is a typical corporate identity for cooperatives in many different fields. Plenty of self-managing societies will have incorporated as an IPS under the NSALG's model rules, though I doubt they'd see themselves as "cooperatives" in the wider sense. Are you talking about sites run as a collaborative collective, like a kibbutz?