Right I’ve worked it out, looks like the value is getting physically updated but it doesn’t visually update in the behaviour tree therefore giving the impression that the blackboard value is not being updated when it actually is.
So basically the service does update the blackboard values as intended, it just doesn’t show it until the task is finished 
Not sure if that’s the intended behaviour or not…
I probably didn’t explain myself very well which hasn’t helped, I also just realised I missed something very obvious in my initial testing…print the value of what the actual blackboard is, not just look at what the screen is telling me
So here’s some screenshots to explain the problem and an explanation of the service and task blueprints.
I’ve had to put it as a separate comment as I ran out of characters