I have just unwittingly stuffed up something in my linux install
I dont really know what that means so Im going to assume you cant boot, right?
Also you should include what distro you are using and any kind of errors you receive. Since without some basic info like that one can only guess.
So, if you cant boot then you obtain some form of rescue disk. For instance if you are using redhat then you can use it and type linux rescue(i believe) and then mount your drives and copy files. With most distros you can use the install disks as rescue disks so try that.