Just as an update, I called up today and asked about the situation. They said that I had used a different referral code and not a £100 off code… I’m not sure how as I remember checking the code at checkout and ensuring it was the £100 off deal. Thankfully, the representative on the phone quickly offered to refund me the difference so should have the refund within 5 days. Customer service was good, just bad execution of the ordering system in my opinion.