ground cover plants?

Poached egg plant

sorry I missed this......... I presume it's a sunny site?

Purple thyme is easy from seed, as are chives, garlic chives and pot marigold of course. There's pink Arabis ..............erm............Stachys Lanata (Lambs Tongues).......erm..........any of the rockery sedums or campanulas .

Anthemis there's a pretty single white that looks like ox eye daisies or the yellow double one  

I love the creeping pholoxes - not much grows through that mat  Same with snow-in-summer and some of the sempervivem creepers. I've also started growing the small hens and chickens for ground cover and they're loving their new homes. Just 'mulch' with pea gravel to keep them from getting soggy on the leaves. Candy tuft is another that will creep though a bit taller but not by much. There's also a creeping zinnia that's supposed to be gorgeous. And of course, sweet woodruff. The latter can get pushy but just grab a handful and yank it out and all is under control again.

I'd really think carefully about lemon balm; had it once and it was a beast to get rid of as it tried to take over the world.

I'd recommend the rockery campanulas too - there are many varieties but the one I've got forms nicely rounded and mounded low clumps about 18" in diameter, with blue flowers quite early in the season. The foliage is light green (and evergreen.)

Can't say I wouldn't have lemon balm in my garden because I have, the main reason being that it seeds all over the place and is very hard to get rid of (I suppose you could say that it certainly does cover the ground, although possibly not just the ground you want to have covered!) 

Alchemilla mollis (lady's mantle)?  As long as you cut off the flowers before they go to seed it won't invade too much & it used to be used in herbal medicine so  there is a kind of link to the herb garden. What about everbearing/alpine strawberries?