With the display working, ive managed to make much more progress with the Dew Heater, but then sadly come to a stop.
The DHT22 humidity/temp sensor is now connected and working fine. The display now reflecting this data,
Unfortunately, it seems my DS18B20 temp sensor is duff. A later update to the sketch from the author also includes test sketches that return the data from the sensors via the serial link to the PC, and this shows error for the DS18. A meter check shows just 0.75V on the signal line for the parasitic power, when it should be around 4V, as it is if I pull the signal line and check it open circuit.
I dont have another of these sensors, so will have to wait now until spares arrive, however long that takes!
I have also discovered that the OLED display will run quite happily with the DC and CS pins open circuit, which saves a couple of wire links.
So it looks like thats it for this project for a while - lucky theres no clear skies again!