You might want to try this lot first
role1.html if they cant help you they might reccomend someone who can.
If you do wind up killing it I recomend that you grasp it firmly arround the wings so that it cant flap or struggle and talk calmly and soothingly to it as you would to a baby. Then hit it in the base of the neck with something heavy such as a crowbar , or road pin - death will be instantaneous. If you are not up for this you coulsd also take it to the local vet who will put it down by lethal injection for a small fee