I'm developing a wildlife garden, so have read up a lot about this. I'm going for mostly native species as they are easier as the conditions will be right and there are many animals and insects that depend on specific plants, so you will be helping them.
Do you have space for a small pond? Or even a bird bath? water is essential. Make sure if it's a birdbath that there is nowhere near it for cats to hide behind. If you build a pond - make sure you have a gentle slope down to it so animals have easy access to it and can climb out easily if they fall in.
For shrubs - choose native british ones, preferably with berries for winter bird food. Climbers also offer good cover and if on a north-facing wall, put in a few bird boxes, and bird feeders (as long as it's not accessible to cats and squirrels). Birds like the cover of shrubs or climbers where they feed. Buddleiha is the obvious choice for butterflies.
You can also buy or make boxes/homes for beneficial insects. Or a log pile is great - allows insects and frogs space to hide as they gradually rot down.
I've found the following website very useful for research on wild flowers and ordering plants/seeds
http://www.wildflower.org.uk.
And on
http://www.nhm.ac.uk/science/projects/fff/ you can look at what plants naturally grow in your area.
To make it look 'natural' stick to asymmetric, sweeping curves. No straight lines. Don't be too tidy - insects and small animals like having dead leaves, etc around for nesting and hiding. Don't use chemicals! Most native plants prefer poor soil, so don't add any fertiliser (makes the gardener's life a lot easier!)
My bible is The Daily Telegraph's Wildlife Garden book.
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