So, I have managed to pick some hard and a couple of very hard locked chests, but others that are ‘very hard’ I am unable to as it says the lock is too hard…

Is there a scale of very hard, or does the game allow you to open main quest linked locks before you would be capable for others?

I found it odd that I was also able to pick only “some” Very Hard chests, but at least 80% of the ones I’ve found, I couldn’t lock pick. So maybe you’re right about a “scale” of difficulty, and/or some Story chests may be harder than others (or easier?).

While I can’t answer your question fully, I know that at lvl 15 lockpicking, you should be able to unlock any Very Hard chest. Since I’ve reached level 15, there’s not been a Very Hard chest that I wasn’t allowed to open. With this theory, as long as you reach lvl 15 lockpicking, I’d wager that you can open story chests regardless.