you can do two ways, sew from seed straight into the garden or propagate them, can't remember what month to do either though, im sure someone else will know!

Around April I sow the seed and then plant outdoors end May. They start flowering mid-July onwards. They produce seeds in pods that look similar to peapods and smell lovely.You can train them up trellis and the like. I let them grow all over the border out of laziness this year and they have done ok if not looking very tidy.

I can get away with sowing in March and I will just mention I sow mine in long thin tubes to give the roots room and to avoid root damage when transplanting.

i've left mine this year to do what they want and they went up and across, look lovely with all their flowers, mixed lot this year for me.

Our local Radio Gardener,sows a lot of his in Autumn,but leaves them in a cold frame over winter.he says they are hardy...never tried it myself,but it seems to work for him ..

I did that the year before last Bloomer and gained nothing in time and some rather leggy plants which succumbed to mould.

As to whether they come up every year, there is a perennial sweet pea (bright mauve, not fragrant) but the usual sweet peas (many colours and some double) are annuals.

For me they're best sown indoors (in the cast-off cardboard inners of toilet rolls is good!) and then planted out - otherwise mice and slugs devastate the seeds and/or the emerging seedlings.

To answer Drc with regards to plants becoming leggy, it's mainly due to them not being grown hard enough.
They appreciate early growth as cold as possible.
Even later sown plants can get leggy if they are grown in a greenhouse or window sills, etc.

This is how I grow them

http://www.growsonyou.com/scrumpygraham/blog/13414-sweet-peas

The compost used is an important factor. I only use peat based composts from mail order that others won't be able to get, but I'd say grow them in John Innes no2. Don't touch garden centre reduced peat composts. I found that growing in 6" pots was a lot easier. The ones I grow in rootrainers are the ones I give away to friends.

Thanks Sg yes I had come to the conclusion that over wintering in my green house didnt work and sowing in pots outdoors in March does. Must admit though I always thought I got away with March sowing because of the mild SE not because it was the cold, so thats a surprise.

In my cold frame, the compost was frozen quite often during the winter. In the worst spell it was solid for about 5 days. They also were covered in snow on a few occasions.
They do recover, but as I said, I wouldn't risk them outside at temperatures below -5 without protection.
And -10, i'd bring them into the greenhouse.

Thanks SG

Grew mine this year,started in pots in house then followed instructions/advise from GOY members. Have had a wonderfull display of flowers. Am leaving them now to go to seed for next year :o)

GW had a slot about growing Sweet Pea and compared planting over winter, March and direct sow in May. Monty thought that he got double the amount of flowers from the winter sown, though March sown was about the same size and might give the same amount of flowers over the next month (possibly). Direct sown gave no flowers yet and were 1/3 the size. If you have the seed then why not try sowing in a few months then some in March/April?

I don't ever watch GW, but what Monty said is common knowledge amongst sweet pea growers and has been for thousands of years :)
Overwintered plants flower longer and have longer stems.
If you can't overwinter, don't sow later than 1st March.
1st February will be much better.

I gave my method, here's another one from the professionals.

http://somersetsweetpeas.com/Somerset%20Sweet%20Peas%20Cultivation%20Page.html

Thanks guys for all the info,il let you know how i get on :-)