I'm tempted to say to raise this if/when you're offered the job, but from personal experience it's best to bring it up beforehand, via the agency.
I went for an interview for a FT job last year, and asked about PT or Flexible Working during the interview. I didn't get the job, and the interviewers gave very negative feedback to the agency. They said that I was clearly time-wasting as I only wanted to work PT, which was not what I'd said at all, as I'd only asked about the possibility. The upshot was that the agency were pissed off as well because I hadn't asked them about PT/FW in advance.
Sometimes leaving it until interview can work, especially if you're head and shoulders above the other candidates, really want the job, and are looking to work only slightly fewer hours than FT, but I'd be very careful about trying it again.
Good luck.