Initially the condition (Day in week Sat Sun) remains showing in use in 'Manage Conditions'. However once I go back in to the rule and click done it changes to not in use.
Hmm - it happened twice. On the original rule that ended up corrupt, I edited my message then deleted the comment in case I was mistaken. However when I've recreated the rule from scratch, the same thing has happened again.
I'll re apply teh condition to the second trigger and see if it 'sticks'