I'd say do whatever you can to make them comfy. Sit & watch their pen for a few mins or more. See if a sound, light or other bird is freaking them out & correct the issue. If they're huddling, they still may be too cold. Sick birds need more heat. Give them a corner with a heat lamp so they can go to it if they need it.
I don't know what you're feeding them other than just crumbles? I'd moisten it with diluted yogurt & water so it's easier for them to digest. I'd also give them grass plugs (dirt clumps) so they can pick at the soil (they love this) & eat the grit.
It's very good to put them on a natural immune boosting diet from the beginning so you're less likely to run into the problems you're having. Do it gradually now though as too much change will stress them. Give them fresh sources of fruit (apples, pears, etc) diced well & mixed into their feed (so they don't notice), give them something to pick at when they're bored (a small head of lettuce or scratch), garlic & oregano are great at boosting immune systems. You can sneak the latter two in (minced) into their feed & build up over a few days. I also give mine paprika, to reduce the chance of parasites, but not much at all - just a couple dashes.
You didn't say how many birds you had, but the pen you mentioned sounds huge. I'd put in a divider to reduce the size so they're not so intimidated by a huge pen. I'd then gradually toss in a nest box or two if you haven't already, talk gently to them (if they are comforted by your voice) so the changes don't upset them.
How tame are yours btw? Like, can you put your hand in there, they hop on & fall asleep?
