its the first time thats hard after that it pretty easy to get off hold the bASE AND JUST PULL LOL
Heres my method:
Unscrew the aerial as far as it goes
Place aerail in horizontal position
Place a thick cloth around the aerial base to protect the roof
Put an open ended spanner between the aerial and the aerial base unit and use the spanner as a leverage - the aerial will just pop out
cant get any signal on any of the aftermarket bee sting style aerials waste of time if you want a good radio reception. unscrewand tug it to get it off.