Posted: 23/03/2014 at 14:07
It will definitely help! Anything like that will be good for helping to break down the clay. Ultimately, it will be the worms that do the work for you and all they need is organic matter in the soil on which to feed. As they burrow through the clay they make nice little drainage holes for you.
PS, just seen the bit about the trench - if you can dig one at the side of the new lawn and fill it with rubble etc., it will help to drain the area. Best put something on top of the rubble to stop the holes filling up with soil though - a sheet of landscape/weed control matting will do the trick.