I Worked It Out
Right, for the purpose of future reference, I'll tell you what I did to get it working.
Didn't spot this before, but you can apply discounting to specific items that meet a certain criteria. In this case I used the calculation of "Based on items with a specific value", using the total of TotalPrice, where DiscExempt has a value of 0, times 0.1 for 10%.
Prior to the above I added a condition where the total number of items with a DiscExempt value of 0 is greater than or equal to 6 and less than 12.
Repeated the process and adjusted the values for 20%.