Another newbie query that I'm struggling with. I'm getting to the point with my pd on raspberry pi where I'd like to use some obscurer externals. I realise I need to re-compile them off the Mac versions so they'll work with my pi zero (arm6) but quick searching I haven't found any definitive instructions for doing so. Any tips?

Your Pi image should include a C compiler that would probably work for Pd externals. Pd-extended's generic Makefile should do the job, if you edit the first lines where you indicate the sources. Katja Vetter's libbuilider Makefile should also work, but I haven't spent time to work with it (I should though).
Give a try with one of those and if you still can't make it come back here.