Posted: 18/11/2016 at 06:54
There was probably a lot of grass seed in the mix. In the warm damp weather it has germinated. If they are just seedlings, hoe them off. If they are the perennial couch grass type, fork it all over again, removing any piece of root you can find. Burn or council bin it. (Do not compost it) Then cover it for winter with thick black plastic or layers of cardboard. When you remove it in spring, it should have warmed the soil and be ready for sowing.