I have had this before couple years back. normally the delivery note is enough. but If you ordered phone online, you will have received an email from ee which lists all the details of your order, normally if you can save as a pdf or screenshot they should accept that. just make sure has retailer logo ee, details of the phone you ordered and the date with in promo dates.
if you can't find email ee should be able to provide you with the order confirmation.
hope helps and not to long winded
