Thanks for the welcome, A2K is an amazing place, the font of all Knowledge.
I'm tempted to leave the flap propped open during the day and then remove the prop during the eveneing. This way Minstrel can go out if she can work it out, but will probably just stay in sleeping in one of my t-shirt draws (her favourite place to sleep at the moment).
The only problem is that our house is not really big enough to have a litter tray lying around all the time.
I'm going to keep on persisting with her, but I don't hold out much hope, she does seem to be a bit thick