I managed to get the original glazier back to look at the unit. It was guaranteed for five years, he said, but since five and a half years have passed his supplier (Pilkington) would not compensate him or me. He then said that the steel frame design was probably to blame (it was a bespoke job, designed by an architect then built to a rather different design by the builder). He added that there was nothing protecting the silicone seal as you would get in most such systems and, because the unit is set back half and inch or so from the face of the frame there was a horizontal cill on which water could gather. South facing, and exposed the the weather, the silicone is likely to have failed, and water penetrated.
He offered to replace the unit at cost with no labour charge, which seemed pretty fair in the circumstances. We discussed applying a bead on the outside to protect the silicone seal from the weather, but this would have to be glued to the steel frame. He concluded that a putty fillet would be the best answer as this would both protect the silicone and throw off the water. I am now waiting for hid estimate.