Ideally, I think, someone would at the allotment at the same time every week, even if it's only for an hour. In my experience, people easily lose track of which are the 1st and 3rd Sundays and are more likely to come along if they know it's every Sunday. However, having said that, I can't help with the rota myself as I'm in Berlin for three months over the summer. Are there others who could take a turn to commit to being there (weather permitting) maybe once a month?