a specimen shrub that is evergreen is the best option here. The dog woods have winter interest in their stem colour, you can get variegated foliage too. Other suitable shrubs could be Eyonomous, elageanus, skimmia [ one of my favs as it is evergreen, bright red berries and fragrant flowers]
I cant think of a perennial herbaceous plant that would fit the bill though

One other question for you first, though - does it get sun and is it open and windy in aspect, or sheltered?

Do any shrubs flower all summer?( eg 3 months) I can't think of any off the top of my head.

If the centre bed is big enough. Perhaps a specimen shrub in the centre with a few perennials or annual plants around it.

I do have two Salvias which flower all summer(!), Cerro posto which is deep pink, and a pale pink one (sorry, can't think of the name at the moment). Also, Viburnum 'mariesii' can flower twice - very pretty white. I like Anchorman's idea . . .

If the specimen shrub was a standard even in a small centre bed you'd have room to plant perennials/annuals around the base and there are lots of these that flower for 3+ months.

A potentilla is the longest flowering shrub I can think of but it has no winter interest as it is deciduous.
If there was a shrub that did all you want I reckon we would all have one! I would go with Anchorman's idea.

In a 4 foot diameter bed you could plant one evergreen standard and around the base plant a mixture of winterflowering heathers and evergreen long flowering perennials. That way you'd have flower/interest all year around

Here's a blog I produced on here on long flowering perennials

http://www.growsonyou.com/anchorman/blog/12681-plants-with-a-very-long-flowering-season

You could also underplant some of the perennials with spring flowering bulbs

Thank you very much for all your comments. I live in the South of England so the climate is pretty good. I will let you know what i do. Thanks gdes

How about hypericum forestii - flowers, evergreen in the south, Semi-evergreen elsewhhere, pink/orange leaves in the autumn with seed heads lasting until spring.