We popped into Scobies (the dry cleaners and launderers) on Wandsworth Common (who also run an exclusive discount for NappyValleyNet users - look out for the next special offers email) and they made the following suggestions...
a) follow the care instructions
b) if the shirt isn't dirty - wash in conditioner not detergent. Less chemicals and the garment will last longer
c) use good detergent - you can tell the difference and always use powder "for whites" when washing whites
d) follow the care instructions
e) only ever wash whites with whites
f) add in a scoop of Vanish "for whites"
g) every now and again put in a "glo-white", but not every time as its a bit strong.
I hope this works, makes me want to go off and find something white in the house to experiment on!
Please do let us know if this helped.
A