One of the staff at Soldebre advised me to mix the roundup with another weedkiller called Stranne.  This has proved more effective than Roundup alone.  Incidentally there are 2 types of Roundup the normal one and one that is 'safe' should sprayed plants be consumed by animals.  We have had to opt for the latter, which is slightly dearer and probably less effective.  If you are interested I will look up the proportions - it is mainly Roundup with a dash of Stranne.