Thanks, David. I agree - no better way to spend an autumn (or winter) day, and today was one of the best.
How many work parties? I didn't really know until I referred to my project listing. It was 24 in 2016, and already 33 in 2017 (obviously most being January - March). If the average attendance is 10, that effort soon stacks up. The volunteers are supplied by Butterfly Conservation, South Downs National Park Authority (their Volunteer Ranger Service and staff) and Brighton Conservation Volunteers.
Elsewhere in Sussex, a huge amount of work is achieved by bodies such as the Murray Downland Trust and Graffham Down Trust. And then there's the National Trust, Wildlife Trust, etc., etc. Where would butterflies, moths and other wildlife be without volunteers? They're the true champions of the British countryside.