Thanks for sharing your problem, I moved this over to the Low Carbon Communities forum section with the hope that it gets more general interest rather than one solely retrofit focussed attention.
That sounds like a challenge, personally as I see it the ideal situation would be to pay people for any work which is being carried out especially if their work is being relied upon and they are under obligation. I understand in certain projects this might not be possible or appropriate. In these situations I think it is really important that anyone carrying our these roles feels properly involved in the planning the work and is able to decide their own schedule and structure where possible. This helps to create a sense of ownership over the project and not just a feeling of being an unpaid worker!
You might also want to consider holding some kind of champion celebration event (in-person events should be possible again soon!). This could be a kind of party or reward for the hard work they are doing, maybe with food an entertainment, something again to make it feel like the champions are not taken for granted.
I am don’t have any specific advice when it comes to getting around the law on these issues, and a lot of it will depend on your specific circumstances!