Jack - don't let the secondment issue sidetrack you. If BALPA win there will be no 'secondments' as such - people will simply be able to bid for PL positions (opening up jobs in BA Mainline). If BA win, their position is that some secondments may be possible a la GSS with all the problems that brings.
However - as far as I read it, this is NOT BALPA's main concern. The numbers of mainline pilots who would want to bid for the far worse PL T&C's is probably very low. BALPA's worry is PL being used as a trojan horse to drive down the T&C's of other BA (& therefore UK) pilots.
If the PL pilots are put on the seniority list it would make NO DIFFERENCE AT ALL to them - other than the ability to join BA mainline after 4/5 years (if they want) and promotion opportunities not being based on how well you get on with your boss.
I am really struggling to see the problems DEP's have with BALPA's stance. In the next 5 years BA will need 'x' new pilots (for mainline and PL). This will be the same whatever the outcome of the talks. There may be slightly fewer DEC's as a result of SFO's from mainline moving to PL for a command - but is the loss of maybe 10 DEC's really worth risking the T&C's of EVERY UK pilot?