In reply to wiggi: I don't think highclimber was neccecerly recommending trangos. Personally, I wouldn't wear b2s up Mont Blanc, but it's certainly possible. September weather can be changeable, last year there was fresh snow about 200m above the valley campsites on somthing like the 1st or 2nd. This year it might still be summer until October. But I would err on the side of warm feet and get some good b3 boots.
If you don't find anything you like in the uk, one plan might be to buy some out there. Never been to Arolla, but I assume you can hire some for a week, get a feel for the boots, try some on and buy out there. Lots of shops in and around Cham for good choice.
The standard summer alpine b3 you'll see lots of is the sportiva Nepal and Nepal evo. Good if they fit, and probably warm enough unless you suffer badly from cold feet or the weather is unseasonably cold. I prefer the newer type of b3 with integrated gaitor, scarpa phantom or sportiva batura etc. Warmer, lighter, more comfortable in my opinion.