Well, i've used this method to sort out my multiple file problem, and it seemed the problem wasn't my files or using the .h file and whatnot - it was that:
A) one function prototype was missing a ";"
B) I forgot to declare namespace std; in my functions source file
But i'll be using this new method from now on, as it looks a lot neater. The error just seems to be another silly thing Dev throws out - it can be pretty silly like that - cheers again.