For what it's worth I believe I was correct in believing a CD4017 had been incorrectly substituted for a 40175. A CD40175 has been installed however the circuit still isn't working
The odd thing is it's a relatively simple circuit in the grand scheme of things, and buffered to the outside world via drive transistors (which check ok). The logic should be purely binary (ie either fully on or fully off). Everything seems to be ok, yet I'm getting some odd logic voltages at a few points.
Substituting the CD40175, CD4093, and CD4049 a number of times suggests that they are not faulty, having said that, the supplier of the 4093/4049 provided them with absolutely zero ESD protection (ie stored in plain plastic drawers and sold in a standard plastic retail bag), so it's plausible they're dud, but seems unlikely to have a series with the same symptoms. In frustration I pulled each of the ICs from the circuit and found pulling the 40175 removed most of the "odd" voltages, however that tube should definitely be ok, as they were from a good source.
I've emailed the manufacturer AGAIN asking for information, but don't expect any joy there, so I've hit a bit of a brick wall here. All a bit frustrating.