I paid on the coach door only a few days ago, driver was very happy to take the money, but it was quite late at night. Seemingly there are other major transport companies which are sometimes inconsistent. That said the machines are right by the Arrivals area so it wouldn't take long to operate them and hop to the bus stop close by. I've only had good experiences on the service, occasionally getting from airside LGW to airside LHR in under an hour. They tend not to get stuck in traffic since there are so many drivers on that part of the M25 that the bush telegraph swings into action to head it off.