Well, the final update is that we have bought 2 new Marathon XRs here in Istanbul. We are not going to mount them immediately but keep checking our tyres for any change in the cuts and replace them if we see any further deterioration.
Schwalbe did get back to me today and say they cannot tell what caused the flaw but believe it was damage on our part.
They have also advised that we buy any tyres we need for our travels from here on in and carry them with us as they cannot really ship tyres to far flung places or at least it will be very expensive. Which brings up the next dilema, carry a full set of tyres for replacement in a few months time, pay up for new ones to be shipped (maybe 100 or more euros for shipping alone to a place like India) or buy what we can find on the spot to get us from say India to Japan. Hummmmm............