Thanks for all the feedback guys! I think the dual neck/leg one will be best for me, and yes - as long as possible.
I'm hoping that by catching them quickly I'll prevent disturbance to the rest of the flock, as at the moment I end up chasing them all round the yard for ages!
They're already numbered and ringed, but we have orf on the farm, and catching them to baktakill is taking a while!