Having accepted a less than RPI rise last time round then we should not accept less than RPI again. Don't forget that anything more than RPI + 0.5% is no longer pensionable. It would have a knock-on affect for the rest of our lives!
If we believe official government RPI figures they can be found
here
They show that we would need an unconditional increase of 6.8% just to stay level with our 2008 real buying power.
If they insist on a multi-year deal then anything less than RPI + 0.5% with no strings attached to help redress the previous loss would still be a slap in the face.