Once the external screws were removed, it resisted all attempts to shift it. I decided that the only way to attack it would be to remove the radio first, then get some leverage at the empty case.
The empty case refused to move, so I decided to cut the aperture to size (larger for the new 601) with the case still in the console so that there would be less surface area and perhaps less adhesion.
I used a Dremmel tool to grind away the Neptune case sleeve, then cut the aperture to size with a jigsaw.
With only the outside edge of the case remaining, I only managed to remove the left-hand part of its external bezel - the rest of it was stuck solid. And it's made of very tough, good quality GRP. I decided to grind through the remaining case bezel at 20mm intervals and remove it with a sharp cold chisel - result!