Can you put up a poster in the library, or other places where mature students are likely to see it: the campus shop, cafes, etc. Does the uni website have a page for mature students where you could put the association's details? Do you have a facebook group? If the SU sends regular emails to all students, you could ask them to highlight your group. Is there a student newspaper? You could write an article about being a mature student there. Even if mature students are less likely to read a student magazine, it helps to raise the general awareness, as it increases the probability that students will hear about it through someone else.