Thank you so much for your replies and friendly welcomes and information.
Thanks to your links I have learned a lot about mites over the last few hours! My adult girls outside have never suffered from mites but now I understand the problem more from your helpful links, I'm concluding that these chicks must have picked them up from a small cage to bring indoors that I borrowed from a friend who keeps hens too. The mites seem to be affecting one of them quite badly but no apparent symptoms on the other 3 chicks yet which is good.
In the past we have hatched fertile eggs under a broody but this time didn't have a broody so have been going down the incubator route for the first time which has been great fun and quite a learning curve!
So I have bought some mites spray powder and sprayed down their cage really thoroughly and plan to introduce them to their first sand/ash bath this evening! I'm going to introduce one for my adult girls too as I didn't realise they might like this. They do have a good go in the dry mud when I let them out to roam around the whole garden a few time a week but I bet they will love that. I'm quite excited to see them when I put it in there and the chicks too.
I'll let you know how it goes and hope to post a picture when I get a mo tomorrow too (they are so sweet!).
I have found this forum so helpful and inspiring whilst reading previous posts and now feel quite grown-up-in-a-poultry-kind-of-way (at 44!) to be writing on here now. I still feel like a beginner even though I just realised that I've been keeping hens for 7 years now and what a great 7 years they have been (no turning back!).
Thanks so much for your help which I really appreciate. I'll let you know how it goes.