I've only ever made marmalade once and it was a spur of the moment thing when i had a glut of oranges i got cheap. The recipe from here that has carrots in it turned out perfectly and my Dad who is a big marmalade snob loved it. I might even make him some for fathers day.
I saw a great tip for the Thermomix (might have been on this forum) - drill a small hole in the flat bit of your spatula big enough to push your thermometer probe through and then use elastic bands to hold the probe to the handle so the tip of the probe lies alongside the blade of the spatula. Now you can easliy test the temperature of your jam as it's mixiing without fear of the probe contacting the moving blades - neat
Pippa, no way will the TMX slice fruit in thin strips. But you can control the size of the chunks by the time and speed you use.