Unless the marketing department is using sloppy phraseology and means the first time Excel and First Choice have flown from CWL to Kos, it raises eyebrows as to what sort of operation they are running.
As mytfly says Airtours/MyTravel flew an overnight Saturday flight to Kos and it operated every summer from 1996 (or even further back possibly) and was only pulled after the 2004 summer season.
It was originally AIH 638 then AIH (later MYT) 762 on the return leg, presumably 637 and 761 on the outbound (I 'inherited' a mass of old records/timetables relating to both CWL and BRS).