Nige if your interested and if you havent already seen them, Wcs1800 current transformers look like just the job for this application, they are basically a torroid like an Ac Ct only with a hall sensor so they'll work at Dc, and being a torroid you just stuff the wire feeding the load through it. They go to 35 amps, so 2 turns would be 17.5a.
I've ordered a couple.
I've ordered a couple.