How are interactions with professors on postgraduate open days taken into account in admissions decisions?
Objective criteria (such as grades, test scores) are often used to make the "first cut" of candidates. At the second and final rounds,  admissions decisions are made holistically and subjectively. 1 It is there that your being proactive and going to the Open Day, talking to professors, and the postgrad admissions tutor become things that  count in your favor. 

How much they will count is uncertain and unknowable. Ultimately it is your suitability to the program that is most important.

1. In the United States, the use of objective criteria such as race  has been cause for lawsuits and scrutiny by the Supreme Court.