Dividend Withholding Tax Refundable ?

Yesterday met people at Seamico office stating it is possible to ask the tax refund for withholding tax on Thai stock dividends. They showed me the form , P.N.D.90.

Is that true ?Considering the deadline for presentation was 31 March , do I risk any fine if I ask for return now ?

I received about 300,000 baht dividends in 2006 so the amount of tax refund would be 30,000 baht. I would of course like to cash that money but want to be sure of not getting into trouble.