The Tax free element of sector pay is controlled by the Inland revenue (or whatever their new name is) and any attempt to change it beyond RPI would have been stopped by them. The company and Balpa have no control over it.
Common does mean the same and is not open to other interpretation from any manager group.
BBow - If I read your post correctly you didn't add 600*£7.80 tax free to your 'offer' figure (assuming the same amount of sectors). Is that not another £4680 tax free on top of the figure you quoted?