Phone in and follow the prompts to "Thinking of leaving us" and you should be able to negotiate some sort of discount. You need to be firm that you really are considering leaving if you don't get a good deal.
Take the usual precautions before holding on for an answer - have supplies of food and drink within reach, use the toilet before calling, etc.
P.S. Don't lean too much on the 18 years, as your loyalty works against you. . They will think you are less likely to leave.
