They say you should always sleep on it. I woke this morning and realised I was approaching this in completely the wrong way. It was never a programatic code issue, it was a math issue, and so...
([Quantity] > [DiscountUnitThreshold]) ? (floor([Quantity] / ([DiscountUnitThreshold] + 1))) * [DiscountAmount] : ''
I guess its another way of doing what Ray has done with my own columns.
Thanks Ray...I will take a look at your method...it might be a little less code :-)