Bugger, I thought it was all going too well. I have belatedly examined the two electrical plug connections on the LH side of the box and spotted that they are in reversed positions, indicating that they affect solenoids in different parts of the box, and also the large round connector is a female receptor on the new box, but male on the old one.
Also even if I swapped the round plug, I would have no idea which wire went to which connection in the plug, although I suppose a Haynes Senator wiring diagram might help IF the colour codes were the same. All in all I think it is a very difficult proposition, which is a great shame to have to lose this brand new box.
New Box Connections
Old (AR25) Box Connections
I think this is probably a deal breaker, as even if I swapped the plugs to connect to the new one, it seems unlikely that it will operate correctly with my ECU. Of course I have the Senator ECU with it, but to integrate that in the Omega is a job too far for me.
Ah well, such is life. Back to plan A and repair the old box with a new thrust washer.