Thank you all for the responses! I'm specifically hunting for A. squamosa, sugar apple. Not interested in cherimoyas, atemoyas or reticulatas, even though they're fantastic fruits in their own right.
I've eaten a lot of sugar apples back in my home country, but I only recall that the seeds were jet black (due to freshness out of the fruit?), plump and smooth. Like @Oolie said these seeds don't really look like they come from a single fruit, especially the flatter & more angular ones in the bag.
They could be reticulatas also. "Custards Apple" If you germinate them you will know soon.
How do I differenciate between the seedlings? Is there a place that details the leaf morphologies for comparison between species? The seller's sending me the correct seeds (mistakes happen), so will be posting those here as well for comparison. Definitely interested to see how the seeds I have on hand germinate too.
The cherimoya seeds I've seen had always been more flat and rougher texture. Those look more like atemoyas and sugar apples because it's more oval, plump, and smooth.
Cherimoya left 2 seeds, Atemoya right 2 seeds.
Thanks for the comparison pic! Yeah my seeds seem to be too smooth and plump to be Cherimoyas proper, but the jury is still out on whether these are Atemoya or Sugar apples.
You should be able to find Cherimoya fruits sold in the UK BTW.
I see a lot of listings using "Sugar Apple", "Custard Apple" and "Sweetsop" here interchangeably. So things get confusing when ordering fresh fruit online.
One site I saw specifically had a listing for Sugar apples, but all the review pictures show that they're cherimoyas instead.