Trying to up my destruction enchantments but it wont let me do any spells on armour etc beyond "destruction spells cost 15% less to cast" how do I up that figure? Im level 100 on enchantments, are there any other perks you need to level up?

Other perks? You didn't mention ANY perks, so... yeah, having any / all 5 levels of "Enchanter" would help, for sure. Certain Alchemy perks can help you make good strong "Fortify Enchanting" potions than you can find as treasure - did you use something like that?

With 60-70 in Enchanting and Alchemy and a handful of perks, I'm turning out gear that boosts my skills by 30% or so, so yeah, I'm pretty sure you can do better than 15%.

Are you using grand souls?

of course - higher level souls give stronger enchanments!