Well here is the culprit. The split was right behind one of the mounting clamps so my theory that a stone or something flipped up and hit it is doubtful. I can only assume that the clamp was with too tight putting stress on it or it was too slack and over the years vibration etc has found a weak spot in the pipe or caused it to micro vibrate against the mounting plate and eventually let go.

maybe other owners of similar spec cars should watch out. Mine has done 120k.
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