Thanks for the feedback everyone!
I'm in Oamaru and I did take the mystery objects back to the Pet Shop yesterday but as the 620 is different to the aquarium they have in the shop I wasn't able to see what I was doing wrong.
Luckily, a friend of my who is very technically minded came to collect an alpaca last night and he and his wife have a similar aquarium. He fiddled around with the elbow and found that a 'connector' had been wedged inside the elbow. I hadn't thought to inspect it that closely! Anyway, the elbow is now firmly attached to the power head and I am one step closer to having my aquarium set up.