plants for dark garden areas

If you're looking for perennials i'd go for Heucheras.
The lime green ones or the amber coloured ones - both would lighten-up and bring the area alive with year round foliage colour.

For something quick and simple you could make an arc-shaped border and plant them in 3s or 5s.

I would take a look at this website. It is Long Acre Plants who also provide a mail order service. Even if you don't buy from them, the selection of plants on there may give you an idea

http://www.plantsforshade.co.uk/

I've used them and it's a good site and good service too.

Ferns are great - large and small!