How many years can I go back to claim SA refund?

I have a client who has never submitted tax returns.  He is now obliged to file them since 10/11 because he is a director of his own limited company.

He has come to me with possible mileage claims relating to previous years going back to 05/06.  As the employer paid below the tax free limits it is likely a refund will be due in each period.  I intend to submit Self Assessment tax returns for the prior years as the refund will be much quicker than if were to try and make a claim using any other method.

How many years can the client go back?  Is it 07/08 because filing date is Jan-09 and we're not 4 years since that date?

thanks for your assistance.