Is it sinful to do non-manual clerical work on Sundays? such as using the time in the day to do reports?
Well it depends on the reports, I guess. It is not only servile works that are prohibited, but judicial acts and commercial occupation.
So reports done for your occupation would likely be prohibited unless they are necessary. You probably shouldn't execute legal documents or purchase property on Sundays and Holy Days.
Another distinction is that the phrase "servile works" is not perfectly synonymous with "manual labor" as it is popularly understood (i.e. physically demanding work). In fact, Aquinas classifies worship of God as a servile work; one which is obligatory on Sundays and Holy Days. The particular definition is found in whom the servile work is meant to serve (God or man).
A further distinction is whether or not the work is "necessary." It is never permitted to do unnecessary servile work on Sundays and Holy Days.
So, to sum up, if the reports are for your commercial occupation you probably shouldn't do it unless it is absolutely necessary. As in, you found out Saturday night they are due at 8am Monday.
If they are for personal use (i.e. you are working on your household budget), I would still avoid it because I don't think accounting would be considered a "liberal work."
Years ago, I was designing our home for construction and architectural drawing is considered by Jone (Moral Theology) to be a liberal work. Even then, Jone warns that a liberal work can become a preoccupation and therefore take away from the purpose of rendering what is owed to God on Sundays and Holy Days.