In case it helps anyone, I managed to sort this myself. Swapped the light units and still had the error message when the working right side unit was connected to the left.
Assumed it was therefore a wiring problem so took off the boot hinge trim and went searching. Quickly found a broken wire at a bent bit of the wiring loom. Fixed the wire and made sure the wiring loom was properly reseated in the trim. 20 minute job and I'm very much a novice.
