A reference to a new iPhone has been found in Apple's Xcode software.
UPDATE: Steve T-S seems to think that this will replace the iPhone 7 as the budget model, with the same screen and processor. This would take the place of the existing 7, or replace the iPhone 6S. It won’t have an iPhone X design.
G. Rambo, a keen developer and someone who is always leaking Apple's products and features, has found a reference to a new iPhone 'iPhone xx', in the Xcode 10 software for developers. This won't necessarily mean the new iPhone will be called XX - that stands for 20 - it's probably just a placeholder.
The new model appears in a list with all other modern iPhones, sorted by alphabetical order. This doesn't show whether the 'iPhone xx' is higher end than the iPhone X, annoyingly.
Upon further investigating, Rambo also found the device to include an Apple A10 processor, that we've seen in the iPhone 7 and 7 Plus. Apple is expected to release a budget model, but it was rumoured to have the A12 in, not a two-year old processor, now much slower than current phones. All three (or four!) iPhones this year will have an iPhone X design.
We're not sure if this means a new iPhone SE 2 - it could do - but Apple will probably unveil this as a press release not at the September or October events. While 'iPhone xx' stands out, this is probably for internal testing and documents and not necessarily an upcoming flagship phone.