OK, took the bull by the horns and pulled the carpet back to have a look-see. In my case (as I was not sure what I was doing and not having the right tools with me) it took me 20 minutes to sort out.
I simply removed the black cover on the antenna (6 screws) inside there is a round white plastic "dome" that you pull towards you (its not screwed in) and the old cord was neatly wrapped up in there. I took it out, put everything back together and followed the instructions to replace the mast and its all done now and working 100%
Its a little tricky to get at as I am right handed and where everything is makes it a bit of a bending act but to make it a bit handier it is easy enough to remove the single 8mm bolt that holds the antenna to the bodywork, and a little cross headed screw that holds an earth wire to allow the unit to be turned around to access the screws.
While in there I noticed that the flexible tube from the LPG filler cap was running through a hole in the body with no grommet on it

So I sorted that out when in there.
So all in all a productive morning with my heater pump and antenna sorted