Funding is complicated and I don’t know all the details. I do know that they get a lot of goodwill from certain suppliers, they also get regular labour from very large organisations for free. They are still getting paid by their respective employers and some treat it as a bit of a jolly. Some of the money comes from things like children in need and the BBC direct, there where about 4 camera and sound guys, 1 H&S guy, couple of runners and the regular cast, who are all salary staff.
On the one I did, probably less than half of the lads where actual volunteers.
I found out about it via a facebook flyer