Hi Peter,
Most decidedly Nipplewort. It has taken around 20 years to get to grips with it. I suspect that the seeds live a long time in the soil and depite removing them each year so that they do not seed they are back again the next year.
If you are unable to dig or pull them out at this time at least try to remove all the seed heads.
I couldn't decide from the pictures which of the many sow thistles and knapweeds it was but will go along with Nipplewort if everyone else is confident. Richard Mabey's Flora Britannica says the name derives from the shape of the flower-buds so if you have a good imagination that should pin the identification down. Perhaps you can control it with a bra.