National Express Terms and Conditions for Ticket Sales online through
Ryanair.com – https://www.ryanair.com/doc/apt/transfer/LGW.pdf
Receiving your Ticket: Please note, after having made a booking in-path through Ryanair.com, you will receive a ticket via email within 24 hours
OK, here’s the answer. Three hours after making my booking I’ve received the following email from National Express:
So the answer is to wait a few hours while they issue a ticket.
Credit:stackoverflow.com‘