sinker said...Get a large pair of pliers they really help to get a good grip on the tab, then pull really hard swearing as much as you like until the tab comes off, them throw the pliers and sail down in disgust and tear the zipper open with your bare hands,
Always works for me..

I had one zip that refused point blank to move.. despite crc, grunting, big pliers etc.. It was beaten in the end though with a pair of vicegrips (hands would probably work too.. but I had more vicegrips than hands at the time

) on each side of the zipper pulling outwards, and another pair moving the zipper itself.
It even still has the tab!
Pulling the stuck part of the zipper outwards seemed to do the tick.