My client has two FMP10 files running on his server — File A which has the data and the relationship graph and B which has some new layouts.

Intuitively I know it must be possible for a script in FileA to go to a layout in FileB, but I can't get my head around how to actually do it. The Goto Layout script step shows only layouts in FileA with no obvious means of presenting current layouts in FileB.

Brain dead this Tuesday morning.

There is no way to do exactly what you asked for. You must create a script in file B that goes to the file B layout. The file B script can be called from file A. Note also that you can make a table occurrence in file A for the tables in file B; and build layouts based on these table occurrences. If done this way there is no need to go to file B.