Quick update:
I continued reading other posts about the same or similar issues and came across a post that states that if you try to access a blackboard before the behaviour tree is initalized it will not work (or words to that effect) (post is here: Accessed none trying to get blackboard - #5 by 7efnawy)
Anyway, as a test added a delay between the BP_Tripod_Controller Begin Play and the function which interacts with the blackboard to see if that makes any different.
This resulted in no accessed none error and it pulled through the correct value from the i_check variable in the blackboard.
Therefore it would seem that I was being a little hasty when trying to interact with a blackboard that wasn’t yet initialized!
Solution: Give Run Behaviour Tree a little time to get its house in order before trying to interact with the blackboard!!
Thanks for your input @Mind-Brain 