More generally, the following are a small sampling of dominant, geographically independent characteristics of Song written vernacular that make me say that it is clearly Mandarin.
1. The standard pronouns are all the same. We have 我, 你, 他, with the appropriate 們 (often also spelled many different ways just as 門 or later on as we get to the Yuan and Ming dynasty 每). Note that unlike the Tang dynasty 他 and 你 are far and away clearly the dominant, geographically-independent pronouns of the standard vernacular.
2. The near demonstrative is 這 (also spelled 者, 遮, or otherwise) and the far demonstrative is 那. While other demonstratives exist (as is the case even in modern Mandarin) these are far and away the most popular, regardless of region.
3. The order of words is effectively always the same as modern Mandarin.
4. The copula is always 是.
5. The various uses of 個 as an auxiliary word apart from just a 量詞 are generally a superset of modern Mandarin.
6. The interrogative for "where" is 哪 (spelled 那) that is paired in all the same ways as in modern Mandarin (那裏, 那個, etc.).
7. The interrogative for "what" is variations of 什麼/甚麼 (e.g. literally those or 甚底 or other things involving 甚).
And so so many other auxiliary words (著, 正, etc.).
In fact, if anything, a general rule of thumb for Song vernacular is that modern Mandarin represents a subset of the usages of various function words in the Song vernacular (e.g. 著 has all the uses it does in modern Mandarin in addition to being something more similar to 被 and other things) and modern Mandarin has simply gotten more "strict" (this is of course a simplification).