Thanks for the advice and apologies to the OP for jumping in. I'll give them one more night and if nothing's happened by morning I'll get on with tackling this membrane. I cut earlier last year and must have managed to get through the sack but this year the veins are away from the egg wall and really close to the hatchling. I'm assuming they'll be ok to be left overnight won't they?
Did you cut that film? If so they should be alright.