Ok - this is doing my head in now.
There are no connectors on the loom floating around that I have missed. If you look at the photo, you can see the '1st gear switch' in question and, looking at the dirt on the plastic, it doesn't look as though there has ever been anything connected to it.
What is even more frustrating is that, although the Haynes manual shows how to change this switch - it doesn't appear anywhere in the wiring diagrams.......
I have only transplanted the bare engine (block and head). Everything else in the engine bay is as it was (manifolds, loom etc). Only the engine has come out, leaving the gearbox, drive etc in the car so there would have been no reason to remove the connector from the 1st gear switch anyway. The gear stick hasn't been touched so there's no reason to believe anything has happened there and this fault was definitely not present before the engine went bang.
View attachment 146514
What am I missing??