And on this point, I did try and group the results for the workflow (1 per customer) but like @KevinJ of Kansas, the workflow still didn't work correctly. I've had some further advice on this today and someone else has suggested a scheduled script as well.
Thanks so much for your responses all - even though I don't appear to have an easy fix (yet!), it's great to get the input from this experienced pool of NetSuite users :-)