Two years ago we planted out loads and loads of sunflowers and since then they have set sown themselves each year. I cannot remember which variety these are, but they are typical, tall sunflowers but multi headed rather than the annoying, in my opinion, Russian Giant, that grows and grows but only has 2 or 3 flowers at the most.
Buy seeds, sow in mid March to early April, keep well watered and plant up into 9cm pots if the weather looks rubbish in May, or if you want to grow them on enough so that they don't get decimated by slugs. then plant them out and watch them grow and by the end of June to mid July, the first flowers will be appearing. Keep cutting the flowers and they will end up flowering on and on until October and the first frosts. I always leave a few of the last flowers to dry out and hang them up for the birds to feast on.
I get my seeds generally from either Higgledy Garden or Chiltern Seeds but you can also find them in garden centres and if you don't remember to sow them in time, most garden centres that stock seedling vegetable plants, will have sunflowers at some point in the year-just keep your eyes open for them!!