a good sharp yank would be my technical description of the action required! :)
 
try twisting from left to right whilst yanking hard and holding the base... if you actully look at the aerial you shold be able to see whats stopping it and then work it out
hope this helps

cheers
 
Its an absolute pain to get the old aerial off but the technique described above does work. I replaced mine with a Citroen C5 aerial, looks loads better and signal reception is not too bad.

You will definitely notice a drop in signal quality, the original Leon aerial is superb at what it was designed for.