Found a solution which works, although it is untidy
Shipping Table
1.1:1.00,2.2:1.75,3.3:2.50,4.4:3.25,5:4.00,6:4.00,7:4.00,8:4.00,9:4.00
Max Package weight in Config set to 10
It works up to the purchase of 9 items, which hopefully is enough!
Would still like to know if there is a proper solution