This indeed worked. The staff working the TTB transfer at M13 at T5 was so confused...She couldn't find the Iberia flight from T3 on her schedule and started googling it...I started getting worried that the incompetence may become a problem, lol... Make sure you have a boarding pass though for the T3 flight, or you will not begetting on that bus. I was the only person on the bus T5-T3, a relatively light route...