I have been with t mobile for a few years, and am out of contract, but i want to change to giffgff which i have been trying for a couple of months and am very happy with, so how do i go about leaving tmobile, is it acceptable to just stop my direct debits to them,or should i phone them and explain that i am leaving.

You should see if T-Mobile will give you a better deal. Otherwise you need to phone them up and tell them you are quitting so they will stop billing you and you will need to get a PAC if you want to keep your number.

Cancelling your Direct Debit is not advisable as T-Mobile may pursue money owed to them from after you cancelled payment.