John,
Thanks very much for your reply. I like your suggestions for the circuit - I will build it exactly like that in the final model. I measured the 5V pin and it read 5.01V so that wasn't the issue. I then decided to measure the resistances again with the multimeter which I have changed the battery in. I got a different reading for the resistance which I thought was really strange given that I had measured it 6 times each with 2 different multimeters.
My measurements are now only out by 0.1V which i will compensate for in code and I suppose i've learned the lesson that my multimeter(s) can never be trusted 100%.
Thanks again,
Jonathan