Help with lawn

Hi, I'm new here and would like some advice please...
Last year we totally re-seeded our garden. it was a major transformation from a large hilly area that only had clumps of grass here and there to three level areas of lovely lawn. One area of grass grew very patchy (with bald spots) despite numerous overseeding efforts. This area of garden is full of clay and fails to drain effectively so maybe this was part of the problem? Anyway, we decided that rather than waste another summer trying in vain to get it going the easiest option would be to bark chip this area so the garden can be enjoyed.
We laid the woodchip 2" thick so it would destroy the patches of grass that did grow and it looked great.
One week later...... The grass is coming through the bark chips! I don't just mean the odd clump that i could pull out either, i'm talking big patches of lawn! So, the advice i need is, how do i kill off the grass without damaging the baby leylandii that i planted around the edge of the lawn? and why has the lawn managed to make its way through 2" of bark chip when it was so hard to get growing before the bark chip went down??:cnfs:

Thanks in advance for any advice offered

 

Mmmm... with hindsight i would have done this but it's a little too late now......

Will this be safe to use near the leylandii? They are only babies (12" to 18" tall and only planted at beginning of summer last year)

Any particular brand of weedkiller work best?