National Express from LHR

I used their service for the exact same trip and was very pleased.

Here are the details listed under FAQ for the National Express regarding your question.

“What should I do if my flight is delayed?

If your flight has been delayed you can amend the date or time of your ticket online using My Account or call us, prior to your coach departure time.

If you miss the booked departure time, you will be required to purchase a new ticket, unless you have our Change & Go add-on which can be chosen when purchasing Airport journeys online.”

Pat,

Yes. I recommend you book the coach that is the earliest time you think you can make but when you proceed to book it, definitely add the flex add on option (change and go). I recently booked it and the added cost was $5. Without adding the flex option, you will likely be out of luck if you miss your assigned departure time.

Here is the info for the flex option Jay suggests
https://www.nationalexpress.com/en/help/change-go

“You are able to board any available coach 12 hours before or after your original departure time.”

I would wager you can make the 9:20 bus. If you have biometric passports entry is very quick. Even if you come in T3 with a long walk, you are already near the transit hub which is between T2 and T3. From T5, just a quick hop over on the tube or Heathrow Express, both free. Even if your plane were an hour late you would be fine. You can always watch the arrival time for your flight during the weeks before your trip, and if you see problems, add the amend your ticket option (which you could also do by purchasing the alter ticket with the option to go earlier.)

Thank you all for your advice. We’re going to purchase the 9:20 am ticket with added flex.