Hang on though, I've just restored it to original and re-tested... it's not picking up the bailed cart again... maybe my test isn't right (adding something to cart, logging in and then closing browser).....

Weird alright, I've had a couple of bailed carts since we started. I'm going to make the change you suggested again and wait until tomorrow to see if any bailed carts show up. If not then it's definitely broken as we usually get about half a dozen a day from signed up customers.