I'm wondering in a general sense when an admissions tutor asks
'what interests you with this studentship' what answers are they looking for. Any important things I must mention?

This is an opportunity for you to show that you have read around the topic and are able to articulate the issues/research needs. I guess that if you have a decent grasp of the state of the knowledge it will be quite easy for you to say why the studentship interests you.

Above all, they are looking for someone who is genuinely interested in the topic because the last thing they need is for the student to drop out when the going gets tough. You having a certain amount of knowledge of what you are likely to be in for will reassure the supervisor that you intend to stay the course.