"Like I said, once you are connected to SQL Server you are on that box, and it doesn't use/know about a .NET connection string to hop over to another server"

Sorry, but I fail to see how this makes sense? When I connect to the listener in my Always On cluster (which is essentially a SQL Instance in a virtual machine) and run the statement "SELECT @@SERVERNAME" against that listener it automatically knows to direct this query to the Primary Replica and returns the server name of said replica... What I want to do is force this to the Secondary Replica. Which has the "Read-Intent Only" property set.

Thanks again for your reply, it is hugely appreciated.