WHBM
Yes National Excess do "bump", even when it's ops normal.... often as not it's because the holds are full of suitcases and the driver decides he can't or won't load anymore - so the bus departs holds full but with empty seats ( another argument for hand baggage only). I've also never seen National Excess staff queue comb to find out who is booked on the "bus just leaving", it's always done first come, first served.
I have to agree with you regarding the train between LGW/LHR - if I could rely on 1 hour for my weekly commute I sure as heck wouldn't use the bus!