So, having thought through the trigger system i've been working out ways to introduce some capacity for adjustment of the angles, but without affecting the rigidity of the arrangement and relative angles to the UV laser beam.

The solution i've come up with is to expand the circular openings into slots, with a slider with perpendicular slot. This allows a small amount of adjustment as the lens of the photo-diode should cope with the off axis beam. There is additional adjustment scope in padding zeros to the beginning of the UV laser data stream.

The image below shows the two photo-diodes (on / off via the SR latch) and the trigger laser diode and collimating lens all facing one of the faces of the polygon mirror.