Hiya Wayne I have gone with a number of tour operators, I beleive that Tereza's are with Portland direct (Although I think this is part if the TUI(Thomsons) group), also Kapetanios is with Thomas cook.
The tour operators do cost more than going independant, except with the situation my family are in. I would go to San Stefanos every year self catering but as there is just my wife and I and one child the costs are prohibitive due to the flight costs. Every year I have looked and every year it is the same, I can book a half board option with the tour operators for only a small amount more than the diy self catering option.
This appears to be a favoutite topic on these boards - the cost of flights to Corfu - the cheapest I have seen for the summer holidays is £890 for the three of us and by the time you add food and accommodation you are looking at nearly £2000. It's time somebody got the cost of flights down , although I realise it's the age old nugget of supply and demand.
All the best.