This is what bluebag was invented for. Find an IGA (not sure if colesworth stock them) and look in the laundry aisle for ‘laundry blue’ - usually a bag or a block. Might be in the cleaning aisle. You wash item, and add the required amount of blueing to the rinse water. Dry as normal. It’s an optical illusion, but makes the yellow tinge disappear. Might be worth considering if napisan or lookalike doesn’t work.
Imo this is the only way to get whiter than whites without nasty chemicals.