Thanks moto,
I've tried having two icons. One containing the script and the other pointing to it with a command line like:-
Writescript HD1:T0/Start-icon
But I get a report that my script icon is not executable, even though it has Executable ticked and its default tool is iconx.
EDIT - I've just put the commands in a script created with EditPad, but this is also reported as not being executable after the Amiga reboots.