The best, strongest sweet peas that I ever grew self seeded from fallen seeds at the end of the Summer. I completely ignored them, presuming that they wouldn't survive.
I'm going to start mine off next week when I get home from my little holiday. I always grow mine in the veg garden so they get watered regularly. I like them as cut flowers in the house so they get picked every day. This year they have been amazing.. I just plant the seeds in rolled up tubes of newspaper and when I plant them out, the whole thing gets planted.
You can start them now in a cold frame making sure that the growing medium isn't water logged . Then , between March until May they need hardening off . I know the theory , but I get mine as seedlings from someone who specialises in growing them.