We've just got back from 2 weeks in Sidari (sob sob) and we hired a car from Dimitri and Manos near The Smugglers and Churchills bars in Canal D' Amour. I believe they also have another branch opposite The Three Little Pigs Taverna. They were very helpful. We paid 140 Euros for an air conditioned 4 door Peugeot 206 for 6 days with mo mileage limit. In my experience the bigger firms who work hand in hand with tour operators charge a lot more because bigger firms are dearer and the t.o want their cut too.
By the way if you intend to go on any excursions use a local agent. We paid an extortionate 75% more to Thomsons for a night trip to Corfu Town. When I checked the prices locally I was furious. Tour operators seem keen to sell excursions at introductory meetings as quickly as possible. They realise that once people have compared prices they won't book with them. The local operators are very helpful and not at all presurising. Hope this helps
Have a great holiday. Sidari is wonderful.