I just received another message, presumably from the ticket that the Twitter team raised on my behalf...

Thank you for letting us know about the difficulties you’re experiencing trying to receive a receipt for your reservation.

I'm afraid I haven't been able to fix this for you at the moment, and I've reported the problem to our systems team who have started working on a solution to correct this. I'll contact you again as soon as we have an update from them.

Please accept our apologies for any inconvenience this may cause, and thank you for your patience. If we can help with anything else please get in touch and we will be happy to help.