Whoops - I was hasty (as usual) and should have kept reading:
(The last paragraph is the key part)

3.2.2.5 Resuming Options
Continue_Trace=bool  Sets continued trace on/off
+C  Sets continued trace on
-C  Sets continued trace off
Create_Ini=file  Generate an INI file to file
Create_Ini=true  Generate file.ini where file is scene name.
Create_Ini=false  Turn off generation of previously set file.ini
+GIfile  Same as Create_Ini=file

If you abort a render while it is in progress or if you used the End_Row option
to end the render prematurely, you can use Continue_Trace=on or +C option to
continue the render later at the point where you left off. This option reads in
the previously generated output file, displays the partial image rendered so
far, then proceeds with the ray-tracing. This option cannot be used if file
output is disabled with Output_to_file=off or -F.

The Continue_Trace option may not work if the Start_Row option has been set to
anything but the top of the file, depending on the output format being used.
Also POV-Ray cannot continue the file once it has been opened and saved again by
any program.

POV-Ray tries to figure out where to resume an interrupted trace by reading any
previously generated data in the specified output file. All file formats contain
the image size, so this will override any image size settings specified. Some
file formats (namely TGA and PNG) also store information about where the file
started (i. e. +SCn and +SRn options), alpha output +UA, and bit-depth +FNn,
which will override these settings. It is up to the user to make sure that all
other options are set the same as the original render.

The Create_Ini option or +GI switch provides an easy way to create an INI file
with all of the rendering options, so you can re-run files with the same
options, or ensure you have all the same options when resuming. This option
creates an INI file with every option set at the value used for that rendering.
This includes default values which you have not specified. For example if you
run POV-Ray with...

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