Bank statement requirement

It says on the guidance for a tier 5 yms visa that the bank statements used as evidence must bear the letterhead of the bank or have an official stamp. Does anyone know what that means? I am applying from an overseas country so cannot just get my bank to stamp printed statements and they have informed me that they neither print nor stamp statements anyway. Does anyone know how strict this rule is??

I gather it's strict. If the statements are duplicates or internet printouts they don't accept them unless they're certified by the bank. If they even LOOK like copies they can be rejected.