The General Registrar has not been at Somerset House for years, they moved to St Catherines House and then to the Family Records Centre, Myddleton Street, Islington about 3 years ago.
At the FRC under red cover books look up your years of birth, find your entry and take note of the reference number - District, Volume, Page.
Fill out a red order form - if you are less than 50 years old them proof of identity has to be given.
Cost is �6.50 if posted, but can be rushed over 24 hours (I think �24).