Make sure you've got good quality 8 or 9" grips at a miniumum - I use Irwin vice-grips as i've found the crappy Draper ones just twist and don't always grip very well.
Another trick to try is to get a large engineers hammer and give the face of the bolt (where the splines are) a couple of good hard hits. Make sure you are accurate in doing this and that the hammer face is exactly square when hitting the splined bolt. This should send a shock down the bolt and free whatever is causing it to stick.
Failing this you can try heating the bolt where the thread goes into the head with a blow torch but try and keep the heat local to the bolt or finally cut the head off the bolt with a grinder, remove the spring and timing belt, then use a stud extractor to undo it.