Detailed information about the National Minimum Wage is in this DTI document. (Note that the actual hourly rates are not up to date. The current 'under 18' rate is �3.30 per hour):
http://www.dti.gov.uk/files/file11671.pdf
As has been stated, some apprentices are not entitled to receive the National Minimum Wage but, in order to prevent employers from abusing this, there must be a valid 'contract of apprenticeship' between the 'master' and the apprentice. (See paragraph 29 in that document). If no such contract exists, the employee is not an apprentice and is therefore entitled to receive the NMW. Further advice can be obtained by calling the confidential helpline referred to in that document.
Chris