Firstly, I assume that you're under 22 years of age, otherwise your employer should have been paying you a minimum of �5.35 per hour, even during any probationary period.
If your employer stated that you'd be on 'full pay' after 3 months, and this hasn't occurred, your employer has breached the contract which exists between you. (There'd be an exception to this if you were told that you'd only get full pay when you'd
satisfactorily completed a period of probation. If you failed to 'pass' the probationary period, the employer would then be within their rights to keep you on the lower rate until you meet the required standards).
Assuming that you've not 'failed' a probationary period, your employer should now pay you at the full rate and also make up the difference in your pay from the end of the three month period. If the employer fails to meet their contractual obligation, you should follow the formal grievance procedure, which is explained here:
http://www.direct.gov.uk/en/Employment/Employe es/ResolvingWorkplaceDisputes/DG_10027992
Chris