I'm planning to take the bus RT in late January. National Express encourages passengers to obtain tickets online in advance but the standard tickets are nonrefundable. National Express does not have a provision for late arriving flights though. Should I wait until I arrive and buy my round-trip tickets at a Heathrow Central Bus Station machine (and hope the next departing bus is not sold out)?
If you choose the Fully Flexible fare that is fully refundable until 24 hours before departure.
You can also choose to add the 'Change and Go' option for a further £5 to travel up to 12 hours before or after your booked departure, subject of course to seat availability.
You don't need the £5 extra on your way back. And don't need to book the return leg until you are in the UK.
Buy the change and go option for £5. Wish we did that. I bought bus tickets that had a departure time for three hours after our arrival. We got through passport control in 30 minutes and tried to change the tickets but couldn't because they were non-refundable and now more expensive to buy day-of-departure.
So we sat in a cafe in Terminal 5 near where the bus stops in T5 and waited it out.
I'd caution you to be aware that National Express doesn't have frequent bus service on that route. If you purchase the more flexible tickets AND you don't mind sitting around the station waiting for the next bus, then it works for you. There are several train routes that will take you between Heathrow and Cambridge.