FvM, I think it turns out your assumption was correct. I didn't recall when I replied to your message that the RH % dropped like a stone to 2% when I changed R20 to about 30K. So, I tried increasing the resistance instead and pleasingly the RH value increase. It stays at 2-3% until the resistance reaches roughly 45-50K then starts to rise. I installed a 500K in R20's space and watched as the RH value increased as resistance increased. At around 500K the humidity peaked at 47%, which was at the time just where I needed it. I then replaced the 500K pot with a 1M resistor and the reading maxed out at 50%. Higher resistances make no difference to the max 50%.
I wondered what a different humidity sensor component might do so I took a broken desk weather station and took out the sensor, which appears to have more surface area given over to the sensitive polymer material. This sensor gives a reading roughly 10% higher than the HCZ-H8-B, so I was able to lower the resistance of the pot. On the HCZ-H8-B the port resistance measures 190K (47%) to achieve what I believed to have been the correct room RH at that time, according to my other sensors. I then measured the resistance of the pot with the second more sensitive sensor and that showed 120K, so just over twice the original resistance. I haven't changed R18 (10K) as it's a SMD and although I bridged it to achieve 5K to test a lower resistance at in its position I would be concerned I'd end up damaging the resistor or tracks with a regular soldering iron. I do have a hot air iron as well, if necessary for any further tests. By the way, all resistors are 1% tolerance.
So, I guess the next step is deciding if changing R20 is a sufficient mod on its own or if I need to some more tests. I think I should mention that R20, R22 and R23 are all through-hole type resistors soldered to solder pads rather than through holes, almost as if they designer expected them to need to be changed. I don't know, perhaps not but you can just see two of them in my attached photo in my first message.
Thanks for you continued assistance on this all.
Thank you very much BradtheRed for your message. I hope that will come in very useful once I have the circuit mod-ed to provide an accurate reading.