Yes jonw65, it is indeed Sophia Valters. Being a relatively wealthy woman with many siblings around, I doubt that tiredness was the reason she didn't register these births.
Her husband, (whose death we can't find) was, we think, the servant to a wealthy family from Northmaptonshire who spent the summer season in London (with servants following them). We were wondering if her husband died and she became the lover of rich person who could bend birth registration rules. But as such registration wasn't compulsory, then no rules were bent.
Hadn't thought of looking for Roberts death record (although she didn't register his birth). Perhaps she couldn't get away with not registering that, but would it have the fathers name on?