On closer inspection, (and many photo comparisons) Hackett is nowhere to be found, which means the photo is pre-1970 (which also means no Phil Collins). Tony Banks is at the back, obviously. Tray was right, after many many more photo comparisons, I think Peter Gabriel is actually sitting down in front, and it's Mike Rutherford on the far left. The long, dead straight hair tricked me, until I saw some late 60's photos of Peter with hair similar to that in the picture. After an hour of searching and scouring for more, rarer photos, I have found the man in the hat to be former guitarist Anthony Phillips, and after a further half hour of searching, the man on the far right is short-term drummer John Mayhew. This dates the photo from his time in the band, between August 1969 and August 1970.