I'm back at home and on less than 1mbps Internet (yeah it's fun), unfortunately
snapd's practice to update snaps in the background, silently, is not very user-friendly for those with poor Internet. I think it's a really great feature, but people with poor Internet won't realise what's taking it up.
Anyway, I used
snap changes then
snap task 337 and
snap watch 337 to see what was happening. It was updating the
atom snaps. I used
sudo snap abort 337 and
snapd was still using up my Internet. Why? I stopped the task and no other tasks running and it's still using up Internet long after I stopped that. I'm running no snaps at the moment.