Hi Brendan,
I've had just the same problem with my E30 320 M20. I had no temp or running problems just a constant lose of water. Then all of a sudden there was telling signs of oil in the water and the reverse. So I striped it down thinking it was the head gasket but knowing about the head cracking problems and what to look for(care of BMW tech in the family always handy). No obvious signs of a cracked head were found and no warpage to speak of, so the alot was removed cleaned and replaced with new gaskets, seals and bolts all a must but the head was not skimmed not always needed but you should check it.
All ran fine but you guested it, still losing water. I rechecked the head for cracks and nothing obvious again but being sure there must be a crack I looked long, hard and very closely and there it was the smallest little crack. the car is still running fine with nothing more than normal temps.I'll be fitting a new head very soon.
I think you should have a good look at the head it might be the same problem. After all this is a common problem.
Hope this helps
DoR.