Well, the plot thickens! 20 mins ago I called Three and they confirmed that I will receive my iPhone tomorrow. Then 10 mins ago I received the following text: 'We're sorry. Unfortunately our stock from Apple has been delayed, meaning the delivery of your new iPhone 7 Plus won't be ready for you tomorrow. We're working with Apple to make sure it's with you no later than 10 October. If there are any changes, or if we can deliver your phone sooner, we'll be in touch, so don't worry about contacting us. Thanks for understanding. Three.'
So, back on the phone to Three to be told that is it 'absolutely certain' that I will receive the phone tomorrow!!