Findaproperty is good, but just FYI, our flat was NOT listed there, but it was on Rightmove, so it's worthwhile checking both.
Also FYI, one of the flats I had my eye on for a while on findaproperty, when I happened to be walking by the agency that was listing it, it was displayed in their window as well so I'm sure it was still available. I think findaproperty is pretty good about taking things off their site as soon as they become unavailable. And our flat was off the letting agent's website as soon as our deposit cleared.