gardenlen wrote:
myn advice but don't store grey water at all use it straight away, and
definately i wouldn't recommned you use the same pump for fresh water
as you do for grey water.

It is usually not practical to use grey water immediately. Generally
considered okay to store for up to 24 hours, the Water Authority
are okay with this. So you only need a small holding tank for the
greywater. As far as the pump goes, I see no problem sharing it
between the two tanks, assuming you are only using the rainwater for
your garden.