Follow up from this post.
This lot's been much better (I took the above photo yesterday afternoon). We've had a good amount of rain in the past couple of weeks so it's really taken off. I got Rosie on the case to help as well. She loves running around with her tiny watering can so I've told her she can water it as much as she likes.
The lavender's still about 50/50, I'd say. I'm not sure it likes the position I've chosen for it. It's a bit windy there. I might have to move the pot somewhere a bit more sheltered while it gets established and then I can put it back where I want it. I'm still trying to work out the fine balance between too much and not enough water on that one as well.