Caper spurge, euphorbia lathyrus

And, personally, I'd remove it from your herb bed.

Thank you for the identification, to be honest I thought it was some variety of euphorbia but with your help I was able to look it up.

Thank you also to the gardener who advised me to remove it from my herb bed and after reading about its toxicity I shall definitely dig it up carefully and dispose of it.

Does anyone know how to propagate a white campanula?

The easiest way is by taking basal cutting. Dig up the whole plant and divide it into smaller plants by simply pulling it apart at the roots. You can also propagate them from seeds, but they might not come true.

Many thanks, will try this method, presumably autumn is the best time for this.

Yes autumn set time to divide.