Next developments.
1. I checked the physical movement inside the hole where the reverse switch goes - there is a pin that obviously moves upwards, connects with the extended arm of the switch and makes the circuit so the reverse lights come on. The power for the circuit comes from the under bonnet connector (that definitely works).
2. The new (used) switch arrived and I decided to rig up a "test" under the bonnet before fitting it in the awkward place that it is. I wired it up as it would be wired when connected properly - +ve to blue/ black wire/ -ve to black wire (Photo 1) and turned the ignition on. I double checked the bridge connection worked again and all good. I then connected the metal arm (photo 2) to an earth hoping the switch would work and NOTHING. I repeated the same with my original switch and NOTHING.
Photo 1


Photo 2



I then got thinking once more and realised that I don't really understand how this switch actually works. Most reverse switches have a a ball bearing that is pressed in making the connection - this just has an extended metal arm that does not actually move. With a +ve in and -ve out how does the connection made by the small arm in the MT75 gearbox make the switch actually work??
Can somebody more knowledgeable please enlighten me as I may be testing it incorrectly OR the fault may actually lie with the inside of the gearbox and not the actual switch!!
Thanks