Well, the MTB currently has too stretched out and low a riding position for her, which hurts her back, and I'm reluctant to sink much money into something which is fundamentally not designed for street riding.

I'm really after something I can find super-cheap (the Raleigh I picked up for £11! It'll be a big profit once I sell the wheels etc). I have a Mixte frame that I was building up for her, where I reversed the stem to bring the bars (North Road style) really nice and close for a very upright position. It's a low end French bike from the 70's (a Jacques Anquetil), and I'll likely finish that, but it would be nice for her to experience a really nice light roadbike. I definitely can't spring for a new 650c bike.