...This was inevitable. I have to say that the cult models absolutely exceeded my every expectation, they are gorgeous sculpts and the way that certain models could subtly change poses (purestrains and abberants) was rather clever. All of the heads for the hybrids are separate, allowing for reposing and swapping to a greater or lesser extent which means that it's possible to ensure there aren't any mirror image models in the force.
Things that I learned in the build that may help others:
1) The magus' knife goes behind the staff, not in front of it like mine ended up - doh!
2) The heads are all separate, I ended up doing one of each pair of models' heads in the 'correct' position, then clipped the post off and changed the positioning on the other - in retrospect I wish I'd taken it a step further and done full head swaps between differing bodies (inter-generational, of course) to make them even more distinct.
3) Note that there are two aberrant bodies and two aberrant weapons, and they are interchangeable to make 4 distinct models. I very nearly made the mistake of pairing both 'body 1' with both 'weapon 1', but caught the error before the glue set on the second model. Phew!
I'm likely going to render down the Deathwatch marines for bits for the Heresy so expect to see some of those being re-purposed in the future. For now, it's time to break out the paints and set to work on the new and improved Benevolent Cult of the Four Armed Emperor!