I think I've screwed it up :-(
I got to the Church today and to my surprise it was better lit than I expected and the sun was shining outside and conveniently the stain glass windows diffused the light quite well. In addition, there were windows on only one side and one end of the church - the opposite walls were solid other than a door. Also in my favour was the ceilings and walls were not bright white but more like a bright cream - better than brown! There was no wooden beams either. In addition, the Church was unusual - narrow and quite long with a large stain glass windows at one end. So I used that as a backdrop and asked all the guests to squeeze in beneath it, wall to wall.
However, despite quite impressive "crowd control" if I do say so myself, and squeezing them together like sardines in order to get them all in using my 80mm lens, I was still a long way off. I'd estimate about 20 feet! I did manage to get myself up on a chair, and the camera was mounted on my tripod that I also raised as advised. The light sensor of the flash was directed to the middle of the group.
I bounced the flash at 45 deg up and also left of the crowd (as the windows I mentioned were to the right of them) to bounce the flash across the darker side of the room.
However, in my panic, I had the flash set only to f8 - likewise with the lens which was set to f8 @ 1\125th (EV13). According to the dial, the maximum range is about 15 feet and I assume that's head on - I suspect it is more like 10 when bounced (though WillTW above says the flash compensates for that?)?
So I am expecting a set of prints with a well lit empty middle space and the figures beyond it in total darkness at the back :-(
I have sent the two films (Fuji Acros 100) off to Ilford for development. I guess I'll see what I get. I am not optimistic though.