I have others, Meadow, but this one is a real spreader.

Oh dear … I sympathise, but don’t know a solution … sorry.

How about the good old devils mix of vinegar, washing up liquid and some salt for good measure and spray every time a new shoot comes up. It will weaken the plants and hopefully kill them eventually. Or keep mowing them, same effect. If you haven't tried all that yet. And then there's weedkiller...
My nasty is bindweed, can't get rid of the stuff! So much for my good advice ;)

Thanks all. OH dug loads out and I think we shall have to resort to weedkiller as it reappears, sadly.

Well, Klahanie, it's even mor annoying to know you have planted this thug when you expected it to be a red one! All my named ones are very well behaved..