Hi! scorpy. Why not try out the RHS for best advice.

http://www.rhs.org.uk/advice/profile...asscontrol.asp

Round-up is very powerful stuff, do be careful. Trouble is it's not very
clever, - can't tell a weed from your treasured perennials. And of course
at half strength it will be only half as effective, so you will have to do it twice as many times. ( Of course you could try potato pealings, not kidding, it does
work, - just depends on how many years you want to wait to see the result. And just how many you can eat!!!.
All the very best of luck with your endevours
Heath