I think I may have solved the problem. I just couldn't work out why she was soaking through her nappies when I have been slowly cutting back on her milk.
Ready for the answer......
She's been gluping down bathwater! I didn't realize how much until a couple of nights ago she completely soaked through a disposable with fleece on top (usually bomb proof) and at 1am I had to change her completely. Then I remembered that DH had bathed her and he (and the toots) were laughing at her drinking all the water!
To be honest I have seen her drinking it but thought nothing of it! So we're back on with NN now!