Mounting Marathon Plus can be a biatch, they are notoriously tight but help is at hand.
When you have the tyres off the rim, remove the rim tape, check that spokes are not protruding into the inner, check the valve hole and join for sharp edges and take off with fine emery cloth. Fit velox cloth rim tape and fit a new, quality brand inner tube. It wont be coming out in a long while. Make sure that the valve is well aligned and fitted in the rim, you dont want to stress the inner. Basically, the tyre wont puncture and the main cause of flats will be stress and damage to the inner tube.