Ken did explain to me how he did it
if you compare this pic with a standard mirror, he made up an aluminium disc to fit in the door recess, then cut down the rover stalk and bolted it to the new base. The base replicates the stock two bolt fixing of the rover 100 mirror base. Bit of filler to smooth things off and a lick of paint and it's done!
When I looked, Rover 75 mirrors were quite pricey which put me off, Rover 25 are very similar if not the same so worth investigating too. FYI: the standard ones you have use heated rover 75 'glass' already, they also sit higher than other phantoms, most have problems with clearance between the mirror and the door...