Right, i've spoken with an engineer at a samsung service centre near me - and he said that the 3-button recovery/download modes is what they have to use in order to flash customer handsets with their flashing equipment. So basically, if the 3-button modes don't work, it's a fault, and they would send the phone back to samsung! Having said that, this engineer i spoke to said he hasn't come across this problem as yet. However, he went on to say he's not sure if you could get the handset swapped out purely to access the recovery/download, as customers are not meant to be using this feature!
Samsung Galaxy S5 16gb SM-G900F
Android ver.: 4.4.2
FW Build: XXU1ANG2
Kernel: Stock