Try OTB holidays for guiding. We were there for our honeymoon in May 2009…no biking though as there was too much snow still but we found loads of singletrack around the Findeln village area on the Sunnegga side of the resort where we did alot of walking. The Gornergrat area has some superb riding too plus you can use the train as uplift (You can buy a special bike pass for the lifts).The Rothorn and Blauherd area above Sunnega is also good.
It is a very popular resort for walkers so i personally wouldn’t bother going during July or August. We’ll be stopping there for a couple of days to ride the stuff we found in 2009….but not until early September as part of a 3 week Alps road trip.