Buried in that Daily Mail article linked to on page 6 is this interesting comment, which if correct tends to support the unlatched cowling theory....
"Another passenger however has described how the casing of the left engine started to come away during take off and that the right engine exploded as the plane was coming into land.
He said: 'As the plane started to gather speed on the runway for take-off, the engine casing came loose and started flapping and at the point of take-off it snapped off with a loud bang".
Runways closed at Heathrow Airport after BA plane makes emergency landing | Mail Online