Sorry to hear about you troubles. About £350/400 is right for a cocker. We have quite a few friends who have cockers and it seems that’s the going rate for a working cocker.
Have you tried the RSPCA, if she is dodgy they may know of her if they don’t you should report her it could help your situation if she is convicted.
I hope the pup gets healthy, it sounds like it came away far too young and it wasn’t best looked after whilst it was with her anyway.
I can understand why you want the papers but unless you’re going to stud/hunt/show the dog they don’t really matter but for £350 you should be entitled to them.
Good luck and the pup get better soon.