My symphaties to you. Does she have any other symptoms (eg. reflux, tummy upsets) other than eczema?
I am only asking because my DD had severe eczema when she was younger. Nothing could keep it under control, including very strong steroid creams and long term prophylactic antibiotics (she was under consultant care and they were at lost of how to treat her).
In the end, I found out that she was allergic to cow's milk protein and soya. Apparently it is very common in babies. She was exclusively breastfed at that time and within weeks of me stopping dairy and soya, her eczema cleared up. It was like a miracle. Perhaps this is something you could consider too, especially if she also suffers from other symptoms. Bear in mind though that it could take at least a couple of weeks (in DD's case, 6 weeks!) before you see the effects, so don't give up on first attempt.
The other thing I could suggest is seeing her doctor again and asking for a different antihistamine. Maybe Piriton does not make her sleepy enough. Secondly, if hydrocortisone is not clearing it up, perhaps she needs a stronger steroid cream. It is better to use a stronger steroid cream for just a few days, than to use a milder one long term.
Also, if you are taking her to see the doctor, you could ask if they would allow you to try different types of emollients (best if they have small packs of samples), so that you could work out which ones are best for her.
Lastly, I am sure you already know this, but make sure you apply her moisturiser (dermol) as often as you can throughout the day (& night, if necessary). I always keep a tub of moisturiser by my pillow. So each time I breastfeed, I moisturise DD as well.
It is horrible to see your little one suffer with eczema. Hope you find a solution soon.