Hi Garry,
Welcome to the forum!
I know the 3.4m one CofG is 72mm from the LE at root.
Now, the 5m S2G does not ring a bell at all. Is that not smaller? 4.2m? CofG of that one is 75mm to 80mm from LE at root.
Rosenthal used to make a 1/3 scale one alright (5m).
As usual, check the decalage, the S2G stuff was notorious for being off (but that is valid for any gliders!). 1.5degrees should do it nicely
Hope this helps