In reply to altirando:
The Haute-Alpes is a good choice. The climbing around the Ailefroide is well known but few UK riders realise how good the VTT is. For a mixed holiday it's better to stay down in the main Durance valley, making it more convenient to visit the different areas. The valley itself has many climbing areas.
Uplift and bikeparks can be found at Les Orres, Vars, Serre Chevalier, Montgenevre and in the Queyras. THe surrounding hills and mountains offer hundreds of kilometres of single track.
http://www.alpineaccomodation.com/#/cycling-mtb/4532428542
This site has some useful links and pictures of the local VTT guidebooks. Added benefit is that you pass by l'Alpe d'Huez and 2 Alpes on the journey. Not much climbing but phenominal biking. If you have a non biking driver they can drop you off in Alpe d'Huez at the lift for Pic Lac Blanc 3330m and collect you at Rochetaillee 711m (north of Bourg) on the main road running towards Grenoble. Thats a descent of 2600m, which is a good way to end the holiday