Following my disastrous testing I decided to try rescuing my WeatherPi version 2 PCB. In summary I'd designed the PCB wrongly, and the three IC chips had the wrong connections.
On the top of the PCB I had to break 3 tracks. I tried a mixture of tools from a drill bit, counter sinc, and eventually ended up with a scalpel blade. Working very carefully I was able to scratch away first the green coating, then the copper track, and to be safe some of the actual board material. I was surprised how thick the copper layer was.
On the bottom of the board I had to make 6 breaks. I'd tried to place as many tracks on the bottom side where possible so that I could see them when troubleshooting!
I carefully solder wires to the bottom of the board using the existing pins of the IC's and headers. I had started to use one of the unused header socket pins at the top of the board but then realized it was running at 5 volts rather than 3.3 volts. Hence why 3 wires run to one section and one to another.
I also attempted to unsolder the 40 pin header to replace with a 24 pin header but despite using a desolder iron and desolder braid, I could not release the header. In the end I gave in and resolder it again.
Once complete I checked there were no short circuits in the design and powered it up.
The Pi powered up and could see the three IC's, and also the temperature sensor.
So the PCB version 2 has been rescued but I don't think I'll use the other 9 boards. At this stage I think I'll work on the design and fix the issues and make another attempt at the PCB. Meanwhile I can work on the code to make it all work.