So in a very big nutshell:
The MLG yoke was warped, probably or possibly... (the report is a bit ambivalent) but not definitely during fitting the night before. This compromising the over-centre downlock. This shouldn't have mattered as the system hydraulic pressure also holds the gear down but a prox sensor fault meant there was no hydraulic pressure. Net result being an unlocked gear leg but 3 greens presented to the crew. An amber warning may or may not have been presented in relation to the prox sensor, but even if it had been then there was no action required by the pilots. Add a bit of a crosswind et voila.