Hiya, thanks. We are in Southern Scotland. Daytime roughly 15-20 degrees celcius, nighttime 10 degrees (although was 17 the other night!!).
Mild temperatures, no extremes. And you are probably getting about 15 hours of daylight so probably not molting from the days getting shorter.
They are a year old. Previously we were getting 11 eggs a day from 15 females. Then early July they went down to laying 2-3 a day. Now the past 2 days, no eggs.
I'm more familiar with chickens than ducks but we had ducks decades ago when I was young. From what I've read I'd expect your Silver Appleyards to still be laying well. That history does not sound right, you should have been getting more eggs.
Are they hiding nests on you? They forage far and wide so they could easily be laying and you don't know it. This is a real common cause of us not getting eggs although they are laying. How hard would it be to look for nests? Or lock them up for a few days so they can't hide a nest.
Are critters getting the eggs? Most critters leave evidence behind, eggshells or wet spots. Some do not. I don't think it is snakes. Snakes tend to eat a few then disappear until they digest those, then come back for more. You are way too consistent for it to be snakes.
Canines take the whole egg. If it were a fox I'd expect you to lose some ducks to them, not just eggs. But does a dog or dogs have access? They can eat eggs while leaving the ducks alone.
Could it be a human? They don't leave traces. That does not have to be a stranger, it could be someone really close. People have been known to so things like this as a practical joke but yours has been going on so long I don't think so.
As long as the ducks are acting normal I don't think it would be a disease.
I'm not familiar enough with SA's to know what their laying season is. Do you have a Scotland SA facebook group you could ask if they know what is going on? They'd know your local conditions.
To me, it sounds like they are actually not laying or they are hiding nests on you. But those are only guesses.
Nothing else had changed. So that's 5 months they were laying for. It feels too soon for them to be done!!