Since I had the car apart to replace a headlight bulb, I decided to find out just what was wrong with the HF antenna mount. I managed to remove the mount only having to unfasten the very ends of the side trims and the last two fixings of the head lining, which gave just enough room to squeeze my hand through to retrieve the SO239 fixing.
As expected, the center pin connection had separated. Luckily not the pin sheared, but the soldered connection to the coax. It seems under enough torque the pin can turn, and over time the wire has split.
After recutting the coax and inserting it back into the connector, I managed, somehow, to resolder the center. Then after a fair bit f swearing I finally managed to get the mount back through the hole in the roof and have it stay there without falling back out for long enough for me to get hold of it from the top of the car and get the nut on! A few minutes of the good old antenna fitters rooftop pump pliers dance, and its secure again.
Tested with the 40m antenna, found enough activity to verify it was operational again.