You can buy a weeding knife to remove weeds, including grass, between paving stones though I find the grass pulls out easily enough it is the likes of dandelions that can be a problem. You might want to consider having the gab between your slabs mortared so that nothing can grown.

I doubt weed killer will work on clumps of grass, I bought a great long handled tool for this last year its got a narrow steel bush with a blade on one end. I love it as I dont have to bend down to clear. Its on my photos and on the garden tools page if you want to see it

Use a path weedkiller such as Pathclear or Bayer Path Control - apply to the cracks and gaps in the paving. Contains something that will kill what's there, and prevent regrowth for the season.

thanks guys, the mortar sounds like a good idea, i was thinking of doing the whole patio again as theres not much left! dont know how hard it will be will look into it! i will have a look at your tool now drc t.y :)