Is there a prompt for 'forgotten password' they send you a link to your e-mail address? forgot username. that's usually a normal thing a bank would do or else a contact us number if it''s 365/24/7 online banking
You'll have a card reader. Try guessing your pin on that with your card. 3 attempts and your stuffed. You can't ring the bank without your customer number, 4 digit pin (online, telephone banking) and password. Don't even bother calling them without it unless you want to have them reset and sent by post. Natwest can't do anything over the phone for you (other than sent out new codes).
Your account number is useless in this matter. 25 years a bank manager for Natwest, I should know. Call the bank and ask them to send out a new pin for your card. Ask for your internet banking details to be sent to you (this will be by post). If you can't remember your customer number (D.O.B including 4 digits) then you can't reset your account online nor will customer services be able to give it to you.
You'll have to call your bank, they will send out your details in the post. There will be to letters and each will arrive within days of each other, so you will be unable to access your account on line for about a week. When you get your details, write them down somewhere safe, with your log on detail and password written somewhere separately.