Thanks for all your help and encouragement guys, been a tough week but I have finally managed to pass the project.
Full process and solution here: Caesars Cipher Project Help
Ultimately took a similar approach to you snowmonkey, just coded one step at a time until I was getting near enough the end product then spent some time debugging until the code functioned as needed. It’s by no means the cleanest or most efficient code but it works and I’m happy with that at this stage 