As a rule of thumb, the further away from the score required, the harder it's likely to be.
Appeals are unpredictable, however, and no one can say what your chances would be.
It depends what sort of evidence you can present. This would need to focus on academic ability in particular:http://www.elevenplusexams.co.uk/appeal ... cation#b11
If the school is full after the allocations, or might become oversubscribed as a result of appeals, then it would be critical not to overlook reasons for wanting a place:http://www.elevenplusexams.co.uk/appeal ... -school#c2
Usually the grammar school would have to be on the CAF because you can only appeal if you have applied and been refused.
Alternatively it ought to be possible to do a late change of preference (and then be refused):http://www.elevenplusexams.co.uk/appeals/general#a26
- but this might raise doubts at appeal about how committed you are to wanting a place at the school.