EZY most certainly do over book their flights, I speak from bitter experience.
In a previous incarnation I worked at sea and had at best an intermittent internet connection. At the end of this particular "cruise" I drove northwards to catch the LGW - EDI flight which the company administrator had booked in my name. Imagine my horror as I entered the hall on the Thursday of a Bank Holiday weekend to be faced with more humans than I thought existed, and all slowly trudging towards the orange check-in desks. After having grown old and shaved twice I finally reached the check-in desk to be told; "You haven't booked in on line therefore your seat has been re-allocated". I pointed out that I had spent the previous four weeks over the horizon and out of electronic touch. The yoof couldn't comprehend how it was possible to be without an internet connection and kept repeating the same mantra.
Faced with a growing chorus of harrumphing from those behind me in the queue I grudgingly joined the easy queue specifically for the disenfranchised, the homeless, those who failed to book in on line and general losers. Having put a further inch on my beard I eventually reached the head of the queue to be told; "The next flight's full"
"RIGHT!! That's it, sod you and your orange nonsense I'm off".
I had no idea where I was off to but it certainly did not involve anything remotely orange, or anything that said Fly or Airways.
Nine hours later as I swept up the gravel drive towards Akro Towers I thought to myself; "Well, that went well, just think of all those Enterprise loyalty points I can use the next time I rent a van"
To recap = EZY DO overbook.