This orchid was on my acquisition list for a while in 2010 when I was collecting small angraecoid and other random miniature orchids of all sorts (I still do, but I don't have any left from 5+ years ago). I never did end up getting one but I may still at some point.
Bobchick, your temperatures, especially the daytime highs might have been too low. From what I see online, this is generally an intermediate growing orchid so I'm sure 10C at night is just fine but I would have aimed for highs more in the 20-28C range. It sounds like humidity levels in your tank should have been perfect. The only way I could imagine the humidity being too low is if your tank is completely open. If covered or mostly covered, did you have a small fan or other mechanism to create air circulation? (Gentle) Air circulation in a closed terrarium is an absolute must.
If you weren't already, I would consider using purified water or rain water, especially if you have a small collection of miniature orchids. 1/4 strength fertilizer at every watering or even once a week should work.
Also, how much light does your skylight allow in, especially this time of year? I am guessing you are located in the UK, so it probably does not allow a whole lot of light right now. I'm in Central Florida and have a ghost orchid growing in a west facing window and even here (being the time of year with the shortest days and most clouds) have needed to supplement the light with an old full spectrum bulb i have. If I put this same orchid in the same window, from March-September, it would be cooked to death within a couple hours. I would look into some full-spectrum lighting. But I am not knowledgeable about the subject of lighting, so I would look into that more on the "lighting" or "terrarium" sub-forums here.
I have had MANY orchids gradually turn brown and die on me in the past 19 years (and then there are all the ones that died suddenly
, and I try to learn from these experiences. You should give this species another try, and if nothing else give it a bit more light and higher daytime temperatures. Good luck!