The union negotiated the terms and conditions that you came over on and that a few enjoyed when volunteering...
BASSA never, ever negotiated the current contract they have merely defended it. The current contract is, for all intents and purposes, exactly the same as it was when BA was privatised and as such was 'gifted' as 'market rate' back when the privatisation took place when fuel, aircraft, landing fees etc were cheaper and tickets were far more expensive.
All credit to BASSA for struggling on to keep what they have until now. Unfortunately it makes the fall to current, modern terms and consitions all the more painful.
Shame they didn't negotiate and rationalise a little more evenly over the years.