Well, thanks to everyone who replied. Probably a little too much detail for me right now, but great none the less.

I've now got hold of some PMK from Silverprint and will try it out soon.

One final question - I usually shoot mf with FP4+, Neopan 400 and recently Efke 25, and develop in Rodinal. Which one of these films is going to benefit most from using PMK?