Setting a checkbox in an ABAP list: In an InputScript we need to tick a checkbox that is displayed within an ABAP list. This checkbox doesn't appear in the GuiXT screen element list. Is there a way to rectify this?

Yes, you can use the expression list[row,column], e.g.

Set list[10,1] "X"

You need to know (or have to determine) the row and column of the checkbox. For details see the "Set" command documentation.