Rghto chaps
It seems that a new card (UHS-1 as opposed to the class 10 that was in it ) has sorted the problem.
I still find it bizare that my mac could find and read the card as could my windows PC but Garmin express could not and neither would mapinstall
Any way i am now sorted but it is an expensive way of curing problems that did not otherwise exist except in Garmin world
Is this said old card faulty or can you now use it in other devices with no problems. Just asking as it may save people wasting or throwing out their old card if it is still useable.