I stumbled upon this thread along with your answer, which made me realise I can’t cast to my HUD as you said (because it’s not an actor, right?) and instead connected the ‘other’ part of the Diamond blueprints BeginOverlap as follows, which fixed the issue!
The objects (diamonds) in the scene now correctly disappear and the ten seconds is added to the players time limit.
Thanks for the reply Fen. 