the DRS protocol is agreed in advance of the match. they go with what is available and in the trott decision they had a good ball, hitting the pads, hitting the stumps and nothing on one hotspot or snicko. the third umpire has no other choice but to give out, he is basically ticking boxes agreed by the two nations before the test match.
england greeting about DRS being applied to the letter after moaning so much about india not using it is rank hypocrisy. their attitude about broad not walking also stinks after invoking 'the spirit of the game' when the indians got bell out a couple of years ago.