Hmm, Well Ive reduced that schematic down to the bare basics, ie without all the fluff, as far as my understanding goes. I dont need it to work at low input voltages, so I removed R2, R3 ,R4 and VR1. Instead I will change R6 (Called out as R1/R2 in my schematic to vary the current). However, it might be more beneficial to use the same method via a switch to vary the current as the original author did, as I am finding out that the resistors needed aren't going to be avaiable anyway. For a 4.096V Reference, I would need 204.8 Ohms and 1024 Ohms for 20 and 4mA respectively. I am finding out that the closest value is 205 Ohms and 1020 Ohms with 0.1% Accuracy.
I also removed D1, but Im slightly unsure about *not* keeping it, only because it does serve to bias the negative side slightly to make the op-amp float.