To slightly expand on that post; I've just got off the phone with the Nokia UK shop (who I'm pretty sure aren't actually in the UK judging by the accents). I explained the situation in full, with dates, and that I wanted to re-order, but at the agreed 15% discounted price. The chap I was talking to did so and confirmed the total as £434.14 (inc. express delivery). He went on to say that had I called ten minutes earlier, he wouldn't have been able to help since they didn't have a solution for the discount code problem, but that now they did.

I haven't received a confirmation email yet, but for anyone in a similar situation I'd suggest giving them (another) call.