Thank you very much Joe and Simon! Always good to hear that you're going in the right direction.
The pH only needs to be at the set point once everyday, and is then pumped out to another tank. Given that, i decided that over a period of 30/40 minutes prior to pumping, my current program should be able to account for any increases in pH. The acid or base that will be added to the tank can diffuse slowly so i don't want to end up with the program overdosing the acid and then trying to make up for it by adding large amounts of the base.
Due to the fragile nature of pH electrodes, I've decided to place the probe externally to the tank. By pumping water from the tank to an external pipe and back, and then fitting the pH electrode so the part that takes the reading is inside the pipe, i can take more stable readings, as the water inside the 500 gallon tank is being constantly mixed by the pump. Hopefully this will decrease the time required for the mixing stage.
I also hope this will help anyone looking for a basic pH management system, i looked on the forums for a while and couldn't find anything quite like this with attached ladder diagram.
I'll be recieving the pH signal transmitter shortly and will let you know on the success or lack of it!