I use the National Express bus from LHR to Woking quite often. I went to the Woking stop and quizzed a couple of drivers. I was advised by both drivers to buy a ticket for a later bus than I expected to need on the same day and then ask the driver to let me get on an earlier one. So far this has worked with 100% success rate (a dozen times or so).

Got a bit of grief once when my ticket was for 23.30 and I rocked up about 15.30 but I think that driver, who I have never seen before or since, was just in grief giving mode. None of the other drivers seemed remotely interested. If you print your ticket out small enough I don't think most of them can read the time anyway. Using 2 pages per sheet seems to work.

I don't know if you can read this across to your route or not. The Woking bus is never full (in my experience), the buses are one an hour or more at the moment.