Repairing a TK50 tape drive?

Woah, just stepped over the threshold from reading to contributing - even if it's only a question :-) I've a TK50 drive that I'm trying to get to work with my Vax. After discovering that it didn't work (the tape never wound itself out into the drive) I took off the cover. There appears to be a piece of stiffish tape connected to the central spinny bit (okay, I know stiffish, and spinny bit aren't the most technical of words! :-), but the other end appears to have been chewed off. I presume this would have the task of extracting the tape from the cartridge. Is there anyplace I can get spares for this part (in the UK), or instructions for constructing something similar myself (I've got a few sacrifical tapes while I experiment). I'd be grateful for any ideas. In fact I'm intrigued how the things meant to work! Cheers, Duncan.