Issue with Supplier Scheduling and Supplier Site not in LOV

I'm setting up and testing Supplier Scheduling for my client.  I have appropriate item, blanket purchase order, ASL, sourcing rules and assignments, a valid MDS and an MRP planned from it. All looks good. When I try and build a supplier schedule, from the Build Supplier Schedules Workbench, I notice two things, Firstly, I cannot select a valid Supplier Site from the LOV - it appears to be empty. I get the FRM-41830 message List of Values contain no entries.  I cannot see anything on the supplier site set up that might influence this. I know there has been an