When you posted here (http://www.flytrapcare.com/phpBB3/a-lin ... t8381.html
) last week, you said you were new to this, so I assumed you had just gotten them. It's neither here nor there anymore anyway. You clearly stated now that you've had them since July.
Some sundews are in fact temperate and require dormancy. However, Capensis and aliciae are both tropical and a full temperate dormancy will kill them. The capensis is probably fine as it's sub-tropical and damn near impossible to kill; the round aliciae may be set back for much longer but even that should recover in time unless it froze one night.
When I said direct sunlight, I was primarily talking about the growing season. As long as they are under where the sun should be if not overcast, that's (generally; within reason) OK. But if they're in an area that's bright but won't see any or very little sun even on a clear day, that's most often a problem. I'll explain more in a bit, as I'm sure this just confused you more.
They do need much less light during winter. But they still need some sunlight unless they are consistently between ~33 F (0.8 C) and ~39 F (3.8 C) degrees. In any case you are right, they really don't need much, but ideally
they need about 4 or 5 hours to more closely emulate their natural habitat.
An example is fridge dormancy. Several of my plants sit outdoors under direct sunlight until late November, at which point they are dormant. We get very very cold winters here, so I uproot them and put them in plastic ziploc bags, then stick them in the refrigerator until Spring, occasionally opening the bags to cycle air and ensure I don't have any fungus growing inside. They get 0 hours of sunlight per day in the fridge, because it's 34 F (1.1 C) degrees in there at all times; it's so cold the plants are basically in a coma and don't need any light. The sudden light change doesn't shock them because 1) they're already dormant, and 2) the temperature drop to ~34 degrees F sends them into an even deeper dormancy. In their natural habitat, winter days can approach 50 F (10 C) or more degrees. At those temps, some light is needed, as the amount of direct sunlight they need is directly proportionate to the temp they experience over dormancy. Will flytraps kept dormant in 48F/9C degree temps that get no light die before Spring? I don't know, I've never tried it, but I'd be surprised if they didn't.
A South-facing windowsill in the kitchen is a great place for them during the growing season, but based on the coloration of the purpureas and the look of the flytraps, they weren't getting as much sunlight as they should. 6 hours or more direct sunlight per day is what they need, and that's the floor, not the ceiling. A few overcast days over the course of each week is fine, but it does up the daily sunlight requirements a bit in order for them to thrive and color up nicely.
When you moved them from the windowsill to the conservatory, my best guess right now is that the overall temp difference in the night time was too great and they went into shock. If that's not the cause, then night time temps may be dropping below freezing in there too much. Additionally, the sudden change in lighting put them into further shock since temps are still reaching what, around 50F/10C degrees or so at peak?
Stick to your current plan and see it through. The only other alternatives I see are these:
1) Keep them at the South windowsill year-round. This will only work if temps at that window sill approach or drop below 50 degrees F. To complicate further, if that is the case, the aliciae is going to be very unhappy, but the capensis (being sub-tropical to be exact) would be fine with it until around 45 and below.
2) Add a 6500K fluorescent light or two (the more watts the better - I'd recommend about 50 total or more) and run them 14 hours a day on a timer during the growing season, dropped back to 8 or 10 hours a day during late fall and winter. Keep said fluorescent lights within 6-8 inches from the plants. 4 inches away is even better if you can pull it off, as fluorescent light intensity diminishes exponentially
as the distance away increases.
Growing carnivorous plants is not an exact science; there's a lot of trial and error involved, but we'll continue to help as best we can.
Is that all clear as mud?
Edit: Added Celsius temperature conversions since you're in UK.