Done it!
Unscrewed three screws from inside bonnet (it helps to move the swan neck of the water washer bottle out of the way).
Remove lens unit via the hatch in wheel arch (awkward, I ended up taking out a bolt from the plastic arch liner itself and pulling it aside to make a large enough hole.
Insert new lens unit via wheel arch and screw in bottom screw.
Screw in top screws via bonnet (far offside one almost impossible to locate so I left it out and instead used some mastic and insulating tape to fix the unit place).
For the inner screw I fixed it to screwdriver end with insulating tape which held it in place during location and tightening).