We've booked our own flights to Corfu/San Stef for the last 3 years now, we travel from Birmingham.
Last year we booked early with EasyJet and got a great deal flying from Bristol. As mentioned above, if you book with EasyJet do it as soon as you can and you will save money. They also offer a money back guarantee, so if the flight price reduces below the price you paid they will refund you. Really impressed with EasyJet, smaller plane, but more leg room and very good cabin service.
But, the flight times changed this year, which made Bristol less attractive, so we've returned to Thomsons from Birmingham. Their pricing structure differs from EasyJet and generally the price reduces if you leave it late. But far too risky for us, so we booked a few months ago.
We have used
www.airflights.co.uk, but you generally find most online agencies offer the same or similar pricing. We usually get a price online and then use our local First Choice travel agency to see if they can match/beat the price.