It took a little investigating in the end, because when I asked about this, I didnt receive any replies to my post, let alone confirmation either way. But I have managed to confirm (if at least, for myself) that this does indeed work the way I was hoping, which is a good thing.
To give a short explanation, I was keen to figure out (for example) whether the stats on a two-handed weapon - such as +40% Basic Skill damage - would apply even if the skill that was selected as your Basic Skill used a completely different weapon set (such as dual weapons).
I tested this by noting the attack value of my Dual Wield Basic skill (e.g. 350-450 damage). I then equipped a 2-handed Axe that increased the damage percentage value of my basic skill.
I went to have a look at the dual wield basic skill again and was pleased to discover that the damage value had risen as expected.
I also verified that the cumulative values also apply across the board to effects such as Damage to Vulnerable, close enemies, stunned, etc etc.
Might be a small thing, and some might have already known this - but when I asked, nobody could tell me, so I decided to check myself and report back with my discovery. 