The code is over 400 lines worth, and it does work for reading and writing the main PLC inputs and outputs.

I have figured out my problem though, and answered my own questions.
The E range of hex addresses point to the Input and Output holding registers, which store the values in hex blocks, which are what the sample code use. Being new to PLCs, I had no idea about the extra registers, it wasn't until I went through all the registers in the Address Picker, that I found the E addresses, and everything slotted into place.

I've been out this morning and a command of 01;03;00;00;00;01;..;.. reads DS1, and I've also got the KFlop writing a new value to DS2, so all that's left is to code the KFlop to store the DS1 value where I can read it in Mach, and get it to read a new value from Mach and write it into DS2.