The larvae of this moth feed inside the seed head of Carline Thistle. I was lucky enough to locate these at a coastal site in Kent at the weekend and will hopefully rear them through.
I noticed that all of the affected seed heads would show a small, darkened clump of seeds that betrayed the feeding larva below. Gently teasing the seed head apart revealed the larva feeding on the seeds at the base of the seed head.
If you do go looking for these, wear gloves.