I don't know what to say, so I'll some questions first
1. Is the area in full sun or deep shade, or somewhere in between?
2. Is your soil acid or alkaline
3. Is the area exposed to cold winds?

If you scroll to the bottom of this page and click on G there is a link to gravel gardens which may give you some ideas

When you say evergreen do you mean shrubs or perennials or both.

I love osteospermum,penstemmon and rudbeckia in gravel beds . They're all perennial and evergreen and all flower for many months

Rudbeckia evergreen? Not in my garden.

Platycladus orientalis 'Raffles', and P. o. 'Minima Glauca' are the two showiest and smallest that I know of. Both need full sun and good drainage.

The rudbeckia I grow is evergreen in my garden in North Cambridgeshire.

there are several different types though and I did buy one last year that isn't evergreen but is just showing signs of new growth now.

This is the evergreen one

http://www.flickr.com/photos/31559373@N00/219907356/

I don't know of any Rudbeckia that isn't either an herbaceous perennial or an annual, Anchorman - I looked at your link, but there's no varietal name, so not much help. Penstemon also disappears here, usually permanently, though not always. And there's only one Osteospermum that survives the winter, forgotten its name...
If the area's in sun, and its sheltered, I'd use Festuca amethystina or glauca, being evergreen, and Luzula nivea, Helianthemum, if there's room, smaller Phormium such as Cream Delight if its warm enough.

The rudbeckia I'm talking about is a herbaceous perennial but an evergreen one. Unfortunately I don't have its name as i was given it as a division several years ago. Although the flowering shoots die the rest of the foliage stays evergreen all winter .

Most of the penstemmons I have in my customers gardens are solidly evergreen all year around in North Cambridgeshire although they can be damaged by very hard frosts. One particular one was still in flower on January the 15th this year and had been in perpetual flower from late June last year!

I grow the osteospermum that is fully winter hardy and I know it as as Osteospermum jucundum

http://www.flickr.com/photos/31559373@N00/160221777/

and it is hardy to at least as far north as Bradford,West yorkshire

that's the one, jacundum, I knew it began with a j... thanks Anchorman

Would Liriope muscari 'Silvery Sunproof' be evergreen in County Durham?

Hmm, not sure, Tugbrethil - its a lovely plant, and it certainly remained green this winter here in London, but in other years it can look pretty tatty by January and all/any leaves left need trimming or cutting back. It's so lovely I'd try it anyway though..

Thanks, Bamboo! : )