So after waiting for ages for a reasonably priced Boss PC-2 to show up, it never did by the way.
I bit the bullet and purchased the alternative, a Yamaha ED-10
It is a drum synthesizer similar to the PC-2, it can take audio in to trigger drums, its works great from the Roland 606 trigger outs and also responds to the CV out from the TB303 but in a slightly different way than my Boss HC-2.
Its a pretty sweet bit of kit, it can power another 5 devices from the power in, if you wanted to make a drum kit, and the audio mix in means you could daisy chain a whole bunch of these together.
You can check out the set on Flickr
Here are some diagrams from the user manual to show how a bunch of them might work together and how it puts together sounds