elephant's policy of charging you to make amendments is pants IMO. However you can reduce it's affect by making the changes online. Send them an e-mail, they give you a price and you send them another one confirming it. They don't charge as much this way.
It REALLY annoys me that when I get a courtesy car when my 172 is in the garage, elephant make me pay £15 to insure the courtesy car (1.2 clio) for 2 days use, pff. They should just be glad I'm driving a shitty 1.2 for 2 days of my policy instead of "the beast".