But on the 1939 register Ethel & William are recorded as married.

Not that I'm aware of but I've no idea where she ended up.

Doesn't mean they were!

I've just ordered the birth cert for Ethel Sharratt b.1930 in the hope that it will confirm Edith Mary as her mother.

Also ordered the marriage cert for Ethel Sharratt & John Neilson 1953 to see who Ethel lists as her father (also some info on John Neilson).

As Kate says, it doesn't mean they were, and in this case it's pretty clear they were not. We have the death reg for Ethel recorded with two surnames and also the birth of a child in Q4 1938 also recorded in two surnames.

But there's also the birth of another female child b. Q3 1937 with the same two surnames. However, she is not the one on the 1939 registration and the blanked out one could only be one of those children (female Q3 1937 or male Q4 1938).
Both were registered in the Spilsby District.

I've looked on FreeBMD, Ancestry and FMP but I can't find the 1937 entry. In any case, yes, I agree with what you say otherwise. Did you check neither of the Spilsby children died before Sept 1939?

First quote is a post I made a few days ago about the 1939 register entry.

I found a birth registration for a child under the name Sharratt maiden surname of mother Sharratt with the same first name in the 1939 Register in Southwark for Jan 1937, not two registrations though just the one so if that man in 1939 was her father then he wasn't present if it is the right registration and the right child. I suppose it is possible the mother could have been in London in 1937 working in service.

If the Jan 1937 birth is a child of Ethel's, then there can't be another in Q3 1937. I still can't find the Q3 birth in Spilsby.

Neither can I.

The blanked out one could be anyone - for instance, an evacuee.