I find that a strong roundup mix usually does the trick....use a pressure sprayer to reach those you can cut back. A second application may be needed.

As I work for the BBC and to avoid conflict of interest regarding advertising, I feel that it is my duty to point out that other brands of systemic weedkiller are also available. ;o)

Thank you for you answers. I have a 'systemic weed killer' in the garage so will try that when the lashing rain stops and I get a dry day!!!