We've always used just a Katadyn Hiker Pro (or similar), which filters down to 0.2 microns, without an additional chemical treatment, and have had no trouble.

Thanks. This is exactly the intel I was looking for!

Does anyone have any updated information on this? I leave for my first trip to the Isle at the end of the month. I tend to be the person that over prepares but the pack is getting heavy for a 7 day trip and I would prefer not to have to bring a form of chemical treatment in addition to my platypus ...