Aha! I've just realised what's happened.
As I've completely rewritten the opening chapters, my characters are now in a completely different setting to the original; thus they're liberated from my original conception of them and free to behave as they choose - I have a sense that they've developed as characters since I first met them, but I've been trying to force them back into the original framework.
Now they've been set free they're running riot - hence Myrtle that 'huh?' feeling you describe. But I agree, after my initial :| it's suddenly exciting ... the rewrite is no longer a chore but exhilarating ... I'm off to see what they have in store for me now ...
Thanks both for your thoughts
Andrea