plants which suck up water and some which do not need much sunlight

Bamboo is quite a thirsty plant, but you might have to contain the area it is in as it can be invasive and send up runners everywhere. Some are clump forming and relatively well behaved. If the ground is waterlogged it might not survive though.

For water logged ground or ground that is sometimes waterlogged dogwoods will tend to tolerate this such as C. alba "Sibirica" which is fairly plain during the summer months but has fantastic red stems during the winter.

The ‘Chameleon Plant’, Houttuynia cordata can survive in waterlogged ground . It might suit your other position as well if you keep it permanently moist as it is has colourful leaves and does well in shade.

Be wary of the Houttynia, though, it can be invasive and come up between paving etc. I would sink barriers in around it to contain it - slates maybe?

To get the red stems on the Cornus, you have to cut it hard back in late winter/early spring each year (once it's established).

is it wise to plant in a soak away? the builder next to us always told me off when i put plants too close to them.

Hottuynia .... Aarghh!! It is absolutely foul smelling. Beware!