If you miss an X26 you can take a 285, which doesn't take too much longer, and also gives the option of getting off to catch a train (requires some knowledge of the area otherwise you will be waiting even longer)

Taking the tube from T5 to Hatton Cross will incur a fare, although taking a bus there won't (except for the X26).

There's an indoor seating area at the Central Bus Station.

Unfortunately, the X26 is now my main method of travelling to Heathrow. The 2-3 hours passes relatively quickly if you have a good book or need to do work on your laptop.