Set up a hidden calculation field with a date result type with something like this:
IF ( IsEmpty ( DeliveryDate ) ; Date ( 1 ; 1 ; 2099 ) ; DeliveryDate )
Now sort your records on this hidden field to see blanks sort to the bottom of the list when you sort in ascending order.
Perfect, thanks again Phil, all done and working.
Part of me feels like I should be at the point where I figured that one out myself though.
But I'm always massively grateful for the support.