In MMB, there's a button "View e-ticket receipt"
Scroll down to "Your flight booking", then it's between "Flight details" and "Passenger Details"
Though when I click it, I get...
Error
Sorry, an e-ticket receipt is not available for the method of payment used for this booking.
Please use 'Print/View itinerary' to print details of your journey.
Booking made by Amex, paid with Amex, fortunately the Amex receipt has the ticket number, even though it's a mystery to BA.