Thanks everyone. This has to be the most helpful and knowledgeable forum I've come across. Also without any of the usual "google it" comments.
I'm using the latest PSexec, and all credentials were also encrypted within an SSIS (SQL) package. However, I also have the option to use a proxy account so that I'm not passing any parameters to the PSexec executable. It's working perfectly now and am able to run the required SAS scripts with precedence to other tasks and database scripts.