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Cheap Holiday Websites
Get your flights as cheap as you can - Neckermann or Jetair as you said - but book the hotel separately. For the last couple of years I've used a German website www.holidaycheck.de and the price of the hotels is way cheaper than buying on other hotel comparison websites, and the combined cost of the flights and hotel is way cheaper than any equivalent package you'll find in the travel agents.
I stayed at the Astir Odysseus on Kos last year myself - its a bit in the middle of nowhere and it was a bit windy too but it was a top, top hotel.
Make sure you send me a postcard!
http://www.astirodysseuskos.com/accommodation-tingaki-greece.aspx#
I used trip advisor to look at reviews of places then us them or something like laterooms or booking.com to book. Travel insurance I have whole year policy
http://www.vakantiepiraten.nl/
There is no better deals that these guys can find. Usually it's based on limited dates, or error in fares, but this would be my first check!
If you can fly out of uk then same website: http://www.holidaypirates.com/
I also have an app to track all the deals.