Steve Round up is a glyphosate , well that is the main ingredent and is classed as a systemic , in other words it is absorbed through the leaves and the gets right down to the roots, unlike some other like gramoxone which just kills the foiage. the downfall been it takes aroud 2 to 3 weeks to see results and you do need conditions that suit. ie. no rainfall within a few hours, and prefferably vigourous growing rate of weeds.
it is quite a safe product and does become inactive in contact with soil, but it will kill of your prize plants if their is any drift.
Idealy ,contact your local agrichemical supplier they sell the good stuff, expensive yes but better than the spray bottle you can buy in garden centres.
I used to do crop spraying many moons ago and round up was the best for for that your type of job,
another thing to try is red deisel . I remember we used to spray braken up in scotland with helicopters to control the growth with deisel, and that takes some killing off
Geoff