In reply to fenski:
In the end, ran from the car park of the Aguille du Midi up to le plaz. Then cheated and got the lift up to Flegere (saving my legs a marathon the following weekend) across the grand balcon, up to the col du brevent, across to brevent, and then back down to town under the planplaz lift.
18km in total, and really spectacular run.
Cheers