Just missed my BA flight from T5 because the shuttle drivers don't care about customers and nor do the managers.
Dropped my car with enough time, but none to spare; spoke to three drivers and reception about needing a shuttle urgently to T5. There was a queue of three buses just waiting, engines idling, but they couldn't be bothered to go. I waited more than 15 minutes for transport.
Having missed my flight, I went back to Hertz and spoke to the duty manager - Adam something - who didn't care either. He couldn't even be bothered to ask my name even though I told him I've been President's Circle for more than twenty years. Just mumbled that he'd "speak to the drivers".

Nice attitude, Hertz.

My advice is use AVIS at Terminal 5 - they're in the terminal itself. That's what I'll do next week and the week after and every week until the end of the year whilst flying in and out from GLA.