well i bought a "big tub" from b and q (seriously, it was called a big tub - 6.50)
then went to tesco and bought 8 bottles of value bleach.
left my zytecs in there for 6 days, and it worked a little. not much though.
to really white them, i would need thick bleach. the tesco cheap stuff was 28 per 2 litre bottle.
the domestos was over 2 quid.
the zytecs arent worth 20 quids worth of bleach
so there you go - i recommend big tub and big dom (big bad dom)