Check any recent letters you have had from the NHS, as these should now include your NHS Number.
Contact your previous GP practice and ask them to look it up. To protect your privacy, you may be asked to show a passport, driving license or some other proof of identity.
Alternatively you can ask your local primary care trust (PCT) to look it up for you. You can search for your nearest PCT on NHS Choices or call NHS Direct on 0845 4647. When you phone your local PCT, ask for the department dealing with patient GP registrations. They can give you your NHS Number or direct you to the right person within that PCT.