Ok I soaked the old battery tray in a Bi-carb water bath for several days and though not perfectly clean it was good enough to determine where the 3 wires were attached plus the links, it's certainly different to a standard 4 cell wiring
Here's a pic of the layout.

I had black wire syndrome so all wire and the 7 pin plug were replaced.
I once made a living from electronics repair but acquired brain injury after radiation treatment for a tumor wipe that part of my memory.
I also found NiMh batteries are a little bigger in diameter than standard AA cells and don't fit the battery boxes.
With a Dremel sanding tube and a scapel the NiMh batteries fit now.