If you are talking about BALPA then BALPA members must make it clear to the union that they want them to act upon this. This has to come from a broad spectrum of members.
BALPA are keen to use the slogan 'You are BALPA.' If enough members contact BALPA and request action then BALPA must move. However it needs to be pushed and continually followed up. There has to be some form of petition or balloting of its members to show the degree of support that this has. This would take a great deal of time and effort on behalf of those wishing to push BALPA to publicly denounce PTF and drive a campaign to the airlines, the DofT and the CAA.
So an external group of members must manage an action plan which will lobby the support of current members who wish to see the end of PTF. It is then taken to BALPA who would have no alternative but to act - surely?? If not the words 'hammer, nail and coffin' come to mind - for BALPA and its inaction at the behest of its members