First thing is to use a 400asa film, this will maximise the amount of light captured from the flash. Is there anyone close to where you live that can lend you a Studio type flash? I have a 300w battery powered you could borrow if you are close to Plymouth.

You do not need to use your smallest aperture to cover off the depth of field, with an 80mm lens on 6x6cm format from a distance of about 15ft at f8 you should get a depth of field from about 11.5ft to 21.4ft, although ideally you would have a wide angle lens such as a 50mm where at 12 ft at f5.6 you would have a depth of field covering from 8.5ft to 21ft which should easily cover your group.

You then need to bounce the flash into an umbrella or large white or silver board to provide more average lighting across the group (you could get someone to hold this above the flash at a 45 degree angle) and to cover the AOV of your lens. If you do have a 50mm lens (or wide angle), you should easily be able to cover the distance with your 45CL-4 on manual and bounced into a large reflector of some sort, with the 80mm you may need to do a couple of tests prior to the event.

During my time as a Professional Photographer I used to capture many large groups on my Hasselblad with a 50mm lens including groups well in excess of 200 and I rarely needed to use greater than f8 to cover the group with more than acceptable sharpness.

Good luck.