Hi viz
As you say, I think it's fair that they get paid. You should call into see them to discuss the bill. They presumably sent you a big bill expecting you to negotiate it down.
From your description of the messy case, they probably did almost all of the work quoted for and probably more besides.
Solicitors will often write off this sort of work, if they bill you for another transaction. But as you went elsewhere, they are right to bill you for it separately.
Brendan