Seeing as I had Amazon voucher unused since Christmas, I have taken advantage of this offer and bought a couple of patterns. They came to over £20 (£20.48 to be exact) and so qualified for free delivery .
I'm not sure what is happening with your Amazon order, SureSt1tch, but my discount appeared when I went to check out. My order total showed as £20.48 + £3.99 delivery, but with a promotion of -£9.11, making the final total £15.36. I'm assuming the promotion was calculated as £3.99 (for delivery) and £5.12 (25% of patterns).