Underneath the "frill" at the top there should be some holes and the seeds can be shaken upside down to get them out. However my sister saved me some and took them off the plant when they were too green and the seeds all rotted.

You should collect the pods, with a fair length of stem still attached and put the heads in a paper bag, secure with an elastic band, hang them upside down and give them a shake every now and then. If the pods are that hard, it probably means the seeds are ready, and you should have a good collection quite quickly.

Thank you both for your comments. I will have a go at shaking them and see how we go.

Gattina's method works well for me

And unless you want to grow a field full of poppies, you will only need 1 seed head lol. Derek

Well, I would like more poppies than we have at the moment, but not a field full. Too late for collecting the pods with stems, but will still shake and see what happens. Thanks for your comments.

You don't need the stems really - just the seed heads are all you need. The seeds will just drop out into whatever you put the seed heads in.