Incidently, ${MD_SYSTEM} is established by another environment file prior.
In my scripts I updated the case statements where they are to be applied:
ZZ) EMAIL_WHO=${MD_SYSTEM}_ZZ_EMAIL_SUPPORT ;;
And hey-presto, a quick test of the script & I got my email.
You have just saved me a minimum 8-10 hrs of rewrites and amendments. And makes their application within future scripts high as this will allow me to write more generic scripts for the six environments we have here (1 script to be applied to each environment - sweet).