Self assessment - determinations

Hi,

A new client came to me at the beginning of the year with a single return outstanding from 2006/07.

I submitted a return immediately showing that no tax was due and the penalties were cancelled out.  However, HMRC are sayign that the tax is still due as the return was submitted outside of the 4 year cut off period.

Is there anything we can do to challenge this as it seems ridiculously unfair?

Any help would be greatly appreciated.

Thank you