I'm so sorry for your loss Zara 
He was a once in a lifetime little guy. I'm glad you have so many happy memories with him.Thankyou everyone for the kind words. It is still so surreal.
The flock are all healthy and alert so it is not something they ate or in the air.
Adélie is seemingly doing ok. She foraged for seed with the flock last night, flock called with Gís, but also spends a lot of time snuggling with me. She is back to how she was like 4.5 years ago.
She has been eating and playing with me this morning, and has been sat on my shoulder for quite a while.
Sydney was my partners fave bird. He was my first bird, a gift from my partner. He was where it all started (I had 6 birds because I was searching for a friend for him). He was a perfect bird, thee definition of perfect. He was happy, social (he would greet everyone and anyone so kindly), playful, never bit, liked scritches, was a great size and a wonderful colour, great flyer, he was quiet mostly but would flock call his Pi-Pew.
When me and my partner call each other on the phone or cross each other in the town, we both "flock call" Sydneys Pi-Pew to each other. I don't know if we will keep on doing that. We have done it for so many years, almost a decade!
I am glad I participated in contests here on AA with him, his iconic photo with all the rubber duckies won Shawnas Stockpiling Contest in 2020. He was also Mr April 2021, and will be Mr July 2024.