Adventures in the garden: Sunflowers

I've never grown Sunflowers before or, at least, I can't remember having grown them before. So, I've decided to give them a go this year. My plan is to grow them next to my fence, so that they have support and shelter but also so that they stand out...if they are successful.

I planted ten seeds and all except one have sprouted at different rates. The above picture was taken in May. I have since planted them into the ground, next to my veg patch and by the fence as planned. And here's a little look at how they are progressing.

I must have been a little eager with the watering because some of the leaves have gone yellow. I'm wondering about feeding them to give them some extra help. Are you growing sunflowers this year? 

Do you have any tips to ensure they produce a beautiful, big flower?