I used the Record Macro function to record a macro for inserting a new sheet 
into an existing spreadsheet.  See below for the macro code that was 
generated.  This code works fine if the spreadsheet contains 2 or more 
sheets.  If the spreadsheet contains only 1 sheet, then this code fails 
without giving an error message.

Here is the generated code:
sub InsertSheet
rem ----------------------------------------------------------------------
rem define variables
dim document   as object
dim dispatcher as object
rem ----------------------------------------------------------------------
rem get access to the document
document   = ThisComponent.CurrentController.Frame
dispatcher = createUnoService("com.sun.star.frame.DispatchHelper")

rem ----------------------------------------------------------------------
dim args1(1) as new com.sun.star.beans.PropertyValue
args1(0).Name = "Name"
args1(0).Value = "Sheet2"
args1(1).Name = "Index"
args1(1).Value = 2

dispatcher.executeDispatch(document, ".uno:Insert", "", 0, args1())
end sub