http://www.hmrc.gov.uk/ct/getting-started/new-company/start-up.htm
Registering your new company at Companies House
HMRC uses the information they receive from Companies House to set up a computer record for your company and allocates it a reference number known as a Unique Taxpayer Reference (UTR). They then send form CT41G (Corporation Tax - Information for New Companies) to your company’s registered office. This form includes your company’s UTR so please keep it safe as you will need it to contact HMRC. It also tells you what you need to do if your company has become ‘active’ and suggests other tax implications your company may need to consider.