Looks like Euphoria amigdaloides Mrs Robbiae, notorious for spreading with enthusiasm!
Take care when breaking stems as the white milky juice is caustic so don't get any on your fingers and then touch your eyes...

It's one of the Euphorbias, not sure of the varietal name, but it does tend to take over. If you decide to dig some out, wear gloves, the sap can be highly irritant to the skin on some people.

Many thanks to you both. I have removed loads since it became so prolific (luckily I always use gardening gloves). It's nice to know what it is !