So sorry you are going through this - It is torture wanting them to stop scratching. You are doing everything right as far as moisturising etc is concerned.
DH just reminded me to tell you about trying different moisturisers too - gp always prescribes the cheapest ones first it seems. we went through trying loads, and loads of them seemed to actually irritate!
The one that worked for us in the end was cetraben. Aqueous cream is a definite no-no - it made DS's skin weep, and I later read that it has the same stripping effect as soap.
Oilatum in the bath, and we used to cake him emollient before the bath - god knows how blocked out drains are! :)
We used to worry about the antihistamine, and hardly used it, but I am quite reassured now that it has no damaging effects, and sometimes when his eczema was pretty bad i would give it to him and it would give him some relief.
In his lifetime he has had LOTS of it because he got bad hayfever when he was a toddler, and he has had many allergic reactions - mild, moderate and severe, and it has literally been a lifesaver. Have had to speak to docs many many times about piriton and am completely reassured that it is safe to give so don't worry on that score.
The advice we were given re giving antihistamine for the eczema was to try to stop the 'itch scratch cycle' (the more they itch the more they scratch, never ending battle). So if you give it every night for one week, you might give her some relief, stop her scratching so much, break the cycle and improve her skin a bit. Giving it here and there might not give an noticeable improvement.
Hope this helps x