Ok, I understand how I need to split the top partition:

ALTER PARTITION SCHEME <PartitionScheme>
 NEXT USED [PRIMARY];
GO
ALTER PARTITION FUNCTION <PartitionFunction> 
...

Thank you, that link does help.

I was after someone who had been in a similar situation & therefore wanted to draw upon their experience, a forum seemed the best place...

in case anyone is interested, I solved this by using a try..catch block:

(also configured the agent job to write out to a log file, so we can capture...

That does help, thanks.

To add, this behaviour only appears to be happen in 2008R2, I've tested in a 2012 instance and do not get the same results; the script runs within SQL Agent...