I have the wrong surname

[Just Kia]

I'm not sure where the problem is either.
You seem to have the right line.

My grandfather was adopted and his legally used name was REDSULL however his biological father was a DEVOS and his biological mother was a JEFFREY.
Because I have this info I am able to follow the bloodline. But this didn't make my mother or her sibs a DEVOS.
I also have a David, who was born before his parents married and at various times was a David JARMAN (his biological father's name) or a David Jarman CLARK (his mother's maiden name).
These are just 2 examples, there's more in my tree that's for sure and I bet a lot of people have plenty more too.

In your case Thomas McDILL was (very helpfully) given the surname of his biological father but used (quite legally) the name of his mother and siblings ROSS and kept the McDILL as a middle name, even passing it on through generations.
So you know that until that point you are a ROSS and from that point back on the fathers side you can choose to look for the name McDILL.
I love Scotland for their records, so much info can be found in them.

[silvery]

Say for example Thomas McDill senior had a huge estate that he had left in his will to all his biological greatgrandchildren when they came of age. 

That would mean that I legally had a claim to part of this huge estate and should be living in a castle in the Highlands, but due to the mistake I discovered in the records  I would never be recognised as his great grandchild and totally miss out.

And that is why properly drawn up wills name all beneficiaries, including children.   Thomas McDill senior would probably not be able to name his great-grandchildren, so they would be 'the issue of his children, x y and z, and of any of their children.    And would be protected by the name on the birth certificate. 
Similarly any inheritance to do with the name Ross is protected by the name of the mother.

[jinks]

I have a Charles Fletcher in my direct line , who in the census
is shown as Charles Ashton along with his wife and children
later they reclaim the Fletcher name.

I think the confusion lies with Charles who are brought up by his
maternal Grandparents "The Ashtons" as a child - he is listed
on the census at this point as Charles Fletcher - Grandson.

When he leaves his Grandparents house he must presume that
his parents were unmarried when he was born - incorrectly -
and I presume he found out later in life after marriage that in fact
they were married and after a brief spell of being known as
Charles Ashton resumes his names as Charles Fletcher along
with his son Joseph Fletcher ( my direct ancestor)

Jinks

The birth of Charles was pre registration and I have found the
baptism and marriage via parish registers

[pjbuk007]

Clare

I too agree that there is not an error in the records - the Registrar recorded how the birth was registered.  So there is nothing to be changed. And, as others have explained, you do not have the wrong surname.

I have several people in my tree who knowingly or unknowingly gave the "wrong" surnames in B, M & D certificates.  It is all part of the fun for me in unravelling who did what and to whom...

I call myself a family historian, rather than a genealogist, as I am interested in the family I came from in the past 200 years, and what they did, rather than what my bloodline is.  But there again, my lot all have names which are common, and belonged to the lower orders, so I doubt if I could follow my bloodline very far.

But other people care a good deal about their genetic origins, and I guess they are genealogists. 

[larc]

Hello Clare
My last name is Dill. My paper trail ends in Ireland during the 1600's. I believe that the Dill were originally from SW Scotland and that their name was originally McDill. I am trying to find genetic evidence to determine if I am related to the McDill. Have you considered having your y-dna tested. I recommend    http://www.familytreedna.com/products.aspx
larc