Hi if I enchant robes with fortify armour will it work? Would it stack with alteration spells? Im just thinking that fortify armour is a worthless enchant for my mage.

The Fortify Armor enchantment fortifies your respective armor skill. If your skill level in light armor is 15, for instance, a 15 point fortify light armor enchantment will increase your light armor skill to 30. Even if you were wearing armor, the enchantment does very little to your armor rating. But since you're not wearing any armor, your skill level in armor doesn't affect anything, thus the enchantment wont help you.

In short:

Fortify light armor boosts light armor skill level.

Light armor skill level boosts armor rating of light armor (bracers, boots, etc..).

No light armor equipped, no boost from fortify light armor enchantment.