Had another look at the duff cam cover, wondering if it could be repaired with some araldite, having a poke about around the main crack around number 4 plug the plastic just crumbled away in a largish chunk, looks like its suffered from high heat at some time in its life and caused the plastic to degrade to the point where someone tightening down the bolts too much has caused the cracks to form. The newish condition of the gaskets would support my theory. The age this plastic is now you would be advised to follow the refitting proceedure to the letter, use new gaskets and rubber cushion washers under all the securing bolts and centre securing nuts, the old ones will be well compressed and not up to the job. Do the centre fixings first starting in the middle and work back to front only doing them up hand tight followed by all the outer bolts in a diagonal pattern working from the centre. Once hand tight repeat the sequence and only tighten to 10 newtons if using a torque wrench, if not just snug the fixings up, the gaskets do not need a lot of pressure to provide an oil tight seal. Check them again after running the car for twenty miles and definately so if you spot any oil leaks.