They can claim for a maximum of 4 past tax years (i.e. 2003-2006 if they submit a claim before the end of 2007). If I was doing this then I would just write a letter to Revenue about it explaining the situation, requesting the outstanding relief, including a copy of my P60s for each year and requesting a new P21 balancing statement for each year. If there is outstanding relief to be granted then they should just issue it by cheque along with the updated P21 balancing statements.