Culprit found and rectified. Thank you for your assistance, as you said it had nothing to do with that invalid M600.
I fixed it and I am learning how to work with macros.
I now completely done away with those print start and end custom gcodes and replaced them with macros. Already works a lot better.
Now compiled a filament change macro and testing it. In about 2 hours if all goes well I will know if it acts as required.
Thanks again and keep well, till next time I muck it up 
