Problem solved. Our farmer friend came up a couple of weekends ago and dealt with them all cleanly and swiftly. We were next to him when he did this as I though it important - well we were responsible for them and I didn't want to shy away from their departure. Yes, they do flap about for a short time (each were passed to us to hold until it stopped whilst he dealt with the next), but it was so quick, no mess, no noise and much much faster than a vet's injection. He was also good enough to show us how to remove the breast fillets (not a huge amount there though) and how we could safely dispose of the remains. Not our nicest morning but really informative. Very little strength needed to do this, no brute force, in fact from what I saw, you would probably use more force in tying up the top of a plastic rubbish bag. When it's done properly like this there is no suffering, it is instantaneous.
