Hmm, doesn't your department run a credit-based participant scheme? Well, obviously not as you're now skint! Well I think it depends on what kind of research you do, I work in Cognitve Neuroscience, so rarely need more than 12 pp per study, so I usually use my friends or just beg some undergrads from the common room! You should talk to your dept. administrator though as they must have come across this before. By the way, do you pronounce your name Sara (like Tara) or Sarah?