I have actually considered making her a large litter tray to use in emergencies, but it'd be a pretty permanent fixture and certainly more problematic than beneficial as I'm trying to sell the house at some point in the near future.
...If only I could get her to go the extra six feet into the bathroom and back over the toilet...
She is normally very good with it. The time it annoys me most is when she does it just before I take her out, which is normally very rare anyway. Dogs get very much into a routine and if you are too late, then they will eventually assume you are not coming any time soon so they go if they need to.
I think it's normally only when I am away and her routine gets messed up because the dog sitter isn't being there at quite the right times. I have spoken to her about it and she is going to make some changes to try and resolve it.