I have a lot of self sown things between the flags in my back yard. Violets, cowslips (I spilled some seeds when sowing in a seed tray) wild strawberries, Erigeron Karvinskianus, Meconopsis cambrica, Alchemilla mollis. However, anything I have deliberately TRIED to grow has failed! Except crocuses - C thomasinianus will self sow when happy. If the gaps are wide enough you can poke in some corms in the autumn. The classic for flags is creeping thyme, but it takes a while to establish if there's a lot of foot traffic. But whatever you try, you need to get rid of the weeds first. Also try to id whatever shows up unbidden because, as I said, all the stuff growing in my yard came thereby itself. (NB for US readers - back yard has a totally different meaning over here. I'm not saying I never planted any of the things in my garden:))

I've seen Corsican Mint at several of my local greenhouses. It's got very tiny leaves, and grows along the ground. Supposedly it stands up to light foot traffic. Might want to give it a try!

Here is a link that might be useful: Corsican Mint

Flora gave a nice list of plants. The article linked in below may have some additional ideas. The article is for an "herbal lawn" but the steppables it mentions can probably be used between flagstones as well.

FataMorgana

Here is a link that might be useful: Herb Companion - Herbal Lawns

Mint or rosemary sounds ideal.

The stuff already there I've ID'd as Probably

Dovesfoot cransbill

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Family - Brassicaceae

Common Name - Large Bitter Cress

and the usual Dandylions.

Thanks for the ideas.

madferret - in the UK rosemary is a sizeable hardy shrub and even the prostrate forms would soon bar the way across your yard. It is definitely not a 'steppable'. If you choose mint it needs to be a prostrate form. As I said Thymus serpyllum, our native creeping thyme or hybrids thereof is the classic flagstone plant. If it's a sunny position I'd go for thyme, if shady, mint.

Sorry I meant thyme :0).

I think I'll go with the mint :0)