Hi Annemarie and all,
Having had a quick chat in the office we are not sure if the Council would be up for using a meanwhile lease. As they are already in discussions with developers they may feel that giving this group any legal footing in connection with this building could be a risk to them, but it's always worth asking!
Another way around this is if the group were to approach the council as someone who could be contracted to do the cleanup works. If they were acting as a contractor then the community group would be the ones who would require appropriate insurance which they could look to get for a couple of days. As an agreement the Council could pay them a nominal fee (£1?) and then they could get on with the job as any other contractor going on site would be able to.
Obviously the group would have to get insurance sorted, the council may cry that they need to tender for works, and they may ask for detailed risk assessments. We were also unsure if the group would need to have a legal structure to allow them to act as a contractor with the appropriate insurance.
I can look into this further if people think it might be worth it.