People seem to be split into two groups here:
Tell them then they have to "be kind" and take it down if they don't they're mean.
Don't bother, they won't take it down, I wouldn't.
Op says How is it fair that we sit in darkness to accommodate their heat solutions?
So: How is it fair that their flat gets overheated to accommodate Op's light issues?
That's a similar question just from the other side. A compromise isn't all on one side.
Currently they probably have no idea that their heat solution is causing issues - so they're not being a bad neighbour here.
But Op has no idea how bad the heat situation is. eg. One of my dc is on medication where heat causes her more issues than normal. It's the first year she's been on it, and she's really noticed the difference. So OP might go round and find that there is very good reason why they need to keep the heat down.
So rather than going round and expecting them to do all the compromising, I'd say that the OP should look and see if there are any simple adjustments they can do - blind, move sofa, put same stuff up etc rather than just expecting others to suffer the heat so they don't have to.
If there's genuinely nothing they can do, then a conversation would be the way to go. But the expectation should be that they're working together, whether it's an adjustment (eg if the window is slightly open, does that mean the light's at a different angle so doesn't come in.