Use TravelSupermarket.com and that will find the best prices for you on dates that you choose. It searches both airlines and travel websites, and since you don't actually book through this website, you pay no commission.
I've always found that going direct to the airline works out cheaper. I usually start at manchesterairport.co.uk (cos that's my local) to see who's the cheapest according to them and then go to the airline's site. I've saved a lot doing it this way and you also get the bonus of being able to choose your own seats.