To link your NatWest account you need to enter:
1. Customer Number
This is your date of birth (ddmmyy), followed by a 4-digit number which you enter when logging in to your Online Banking.
This is the 4-digit number you have to know in the step 2 to log in to your Online Banking. NatWest asks for 3 digits, but to link your account to Plum you need to enter the FULL PIN.
This is the 6-20 characters long password which you use when logging in to your Online Banking. NatWest asks for 3 random characters, but Plum requires the FULL password.
If you have forgotten your Online Banking details follow the instructions here.