I had that happening to my old bivi, gonna sound daft but I stood up inside it with it over me like a big French letter! Lol, anyway, once inside I was transfixed by the starry night experience and found loads of tiny pin prick holes all over the foot area. The bag wet out all over anyway and would seep in like yours but especially around the abrasions and tiny holes, in the end tech wash killed it by then stripping all the seam seals of of it, in the bin it went, I've now got a Lithuanian bivi, brand new same as those from endicotts.
I was left feeling a bit let down by the British bivi, it didn't breath very well either, but was very roomy and the materials was nice and soft. I reckon I'll try again with a British one, but only brand new.
"a good reputation is hard won, and easily lost" - my nan