I can't honestly remember where I bought the ticket, but it certainly wasn't at Bromley Garage (TB) as I've never set foot in the place. Buying Rover tickets if you lived miles from the Underground or, alternatively, a bus garage was a perennial problem, as there was no facility for buying them much in advance.
You're right about the 727 - probably wasn't one in the bus station, and the thought of hanging round in Crawley on a very late Autumn Sunday afternoon had no appeal, whereas I daresay there was a 405 lurking ready to take me back to civilisation (well, Croydon, anyway
)