In reply to ChrisP:
Try the British Mountain Guides web site where you can put in a request for a guide, and it's get sent throughout the whole of the BMG membership. There shouldnt be a problem with you doing your own thing first, might be worth letting the guide know what you have planed to get acclimatised as he or she may have some tips and ideas for you.
Cost's up to £300 per day,split between you and your friend, that may not include the guides expenses, hut fee,lifts cost,even though guides get a discount on the lifts, you'd have to have a chat with the guide about that, and also what route you'd like to take,hut bookings can be a pain during busy times.As a guide myself I would also like some info on your past experience which would be very helpfull.Also some companies do run a 3 day Mont Blanc exstension to there trips, so it might be worth a call, to see if you can get on one of those,haveing already done your own acclimatisation.Avoid August if you can,very very busy,and get as fit as you can before hand,Good luck and all the best.Might see you up there.