Seems there is a billion posts on this but not one of them helped me enough to fix this problem in the end i think it came down to blind luck and finding out the relevance of STSDev not building a pdb file unless you build your project with the debug build solution configuration selected.
Anyhow this is how i finally managed to get my debugger working within sharepoint with STSDev.
** for information i have used this gac registry tweak to enable me to see the full gac structure
This finally worked for me and i am now able to debug.