Grimson, yes! I can see how that would be the most sensible way make changes. I did have a go at making patch files with diff, but those I made didn't seem to work out and at this stage I don't want to spend too much time finding out why.
vpeter, this solution works and is ideal for cursory experimentation. I want to make lots of small changes in between builds and see how they affect things and this quick and dirty method does that. It may not be the right way for now, but I can get to that once I have a handle on what's going on.
Thanks guys!