EA's have just rang to say that the offer that had been accepted last week has now been matched by a chain free buyer so the seller has agreed to accept that instead. He didn't ask us to up our offer (not sure we would have done anyway) so it seems genuine.
Not a big deal in the scheme of things (it was a compromise house, but in the great school catcment area) so we'll just keep looking and hoping that something better comes up soon...