Okay folks, bit of a positive update:
I tried increasing the concentration of the blix. I mixed the remaining parts A and B I had to the correct ratio and then added this to the litre of blix I'd mixed up properly (with water).
Everything else remained unchaged (except putting black material over the small light leak - though I'm quite confident this wasn't the issue).
Anyway, with new stronger blix, the blacks are darker and contrast is improved. I'll try to get a couple of examples scanned and uploaded tomorrow when dry.
They're not quite as dark as I'd like them, but I think my issue is with the blix. See attached image for the bottom of the bottle, which I had mixed a bit with a stirrer.