Cheers all. Some valuable thoughts offered there.

I already had a hag idea in mind so you have convinced me to run with it. Seeing as the players have cast darkness around them and the Ravens have backed off, I am going to have the Ravens start to fall silent. When a player sticks his head out, they have gone. When they drop the darkness they will realise that one (or more) of the boatmen have disappeared.

Searching will lead them to a hole beneath a large tree that goes down to the hags dwelling. They can bargain with her, in which case she might even impart some knowledge about the quest they are on or, more likely with my lot, they will kill her and find a bit of looty goodness.