Daveb99's post in High Level Brake Light - LEDs was marked as the answer
To finish this one off ….. I bought a genuine Rover one from eBay and fitted it the other day. Cost £13.50 and I simply cut and soldered the wires, and drilled two holes for the fixing bolts as per the original one.
Straightforward job - and a very cheap fix with the proper part. Thanks for all the help 👍🏻