I wrote a looong reply here yesterday, but then my phone died just as I was about to post it! Thanks for the advice Catriona! That's pretty much how we're doing it at the moment, no punishment for getting it wrong, but lots of praise and some tasty treats when she gets it right. We didn't use to give her treats for it, I think it makes all the difference as it's giving her a reason to do it outside.
Another mistake we've made is to relax about it after a few days of no accidents, she haven't had any for a couple of days now, but I know better than thinking she's housetrained!
Going OTT if she scratches the door again was a good idea too! When she did it I was just so baffled by it I didn't think to do that. It's good to have these things in mind!