As far as I know, the length of sweetpea flowering stems is down to the variety you're growing rather than anything you do to them.

Choose Spencer varieties, start them off late October/early November, overwinter in a cold frame , plant out end of March/early April in well prepared soil. When they start to flower make sure the soil never dries out. That's the only way to get the really long stems.

thanks for the interest, it helps a lot