Thanks for the replies - they've given me a bit of insight into how these work from the teacher's point of view, which is exactly what I wanted.
To give full disclosure I did actually sign up to try these out and had my first lesson on Friday. I came out with very mixed feelings, though obviously one lesson is not enough to decide if it's for me or not, especially as I was nervous and it took half the lesson for my bow to stop shaking!
Although I've been teaching myself for almost 5 years, I've managed to get to the stage where grade 3 pieces are quite 'doable' and I'm staring to tackle grade 4 pieces. However the student I'm paired up with has been playing less than a year and is still working on grade 1 pieces. On Friday we worked on the same piece, with me sight reading it.
Despite what I thought was quite a hectic lesson, I thought the teacher was very good, and I came away with several technical things to work on.
If all the lessons are to be based on grade 1 pieces I don't think I'll continue with them, though it sounds from the replies that isn't necessarily the case.