National bus Heathrow to Bath -- buy in advance?

Book ahead. When you get to the "choose fare" section, there is a Change Coach option, which gives you a 12 hour window before and after your selected time. You may board any coach within that window. Costs 5 pounds extra, if I remember correctly.

Full fare, for instance on two buses remaining to depart this evening, is about 36 pounds.

Should book ahead. Bus can fill up since the total seats are very limited. Not like a train with huge capacities.

The line is never long at the window to buy your bus ticket at Heathrow. We bought ours--just walked up after our flight--this past April. We waited outside the sliding glass doors, our bus to Bath pulled up into one of the covered stalls.
Hopped on board, carried our carry on bags with us. For larger luggage, the bus driver will put it in a closed compartment under the bus.
There were 4 other people onboard our bus from Heathrow to Bath. Lots of vacant seats on the bus. We sat near the front of the bus and chatted with the bus driver. If you want to know the names of the villages and towns you are traveling through, just ask the bus driver.
We arrived at the bus station in Bath, and took the Number 4 city bus to our hotel, Brooks Guesthouse. We were starting our Rick Steves England In 14 Days tour, which we enjoyed very much.
If you are staying in a hotel in downtown Bath, you can walk there from the bus station. It's not far.

It is possible to buy these tickets well in advance at considerable savings. Six months in advance, they were £15.50 each. With a three and a half hour interval between arrival at LHR at Terminal 5 and departure on National Express at the Central Bus Station between Terminal 2 & 3, there was plenty of time to find the bus station, have a nice breakfast, get Oyster tickets for use when returning to London, and still wait comfortably for the coach to arrive.

thanks for the info! One more question -- about how long did the trip take? In the RS book it says it takes about 3.5 hours but the National website says 2.5 hours for a 4:25pm departure.