If you - like Alan Titchmarsh - love sweet pea, you can sow it any time over the next two or three months to bloom next year. You can also wait until next spring, but plants sown now will flower a bit earlier, and may be sturdier. Some gardeners like to soak the seed overnight in warm water after "chipping" it, that is making a nick in the hard coating with a knife or rubbing it with sandpaper.
Sow the seeds half-an-inch deep in pots of compost and keep outside or in a cold frame. In cooler parts of the country, you may need to bring them into an unheated greenhouse during cold spells. Do not expose to artificial heat or you will get sappy, too-tender growth. Pinch out the growing tips when the plants are about four inches high to promote bushiness.