Papaver argemone. Prickly Poppy, this pale scarlet poppy is smaller than the common poppy, flowering from May to July. Looks similar to other poppies but distinguishable by the seed heads which are long, narrow and covered in bristles.
Amazingly the seed will stay in the ground for a long time until the conditions are right for germination. Once the plants are established the soil must be turned occasionally to get further germination. The seed will germinate in the autumn or the spring.