Thistles and Docks

I reseeded some ground last spring and since then it has become infested with Thistles and Docks, I put down a grass and clover mix so there my problem lies how do i get rid of the weeds without killing the clover.
The field was burnt off before disking it, its too shallow to plough. I have since learnt that its better to put down the grass seed first, Treat whatever weeds come up and then shake the clover seed but its no good talking about that now.
Any suggestions greatly appreciated and thanks in advance.