Thanks- erm quick question. I tried to cut the face in half and did a test mirror; when I do it the mirrored half appears some distance away. I have tried recentring the pivot, snapping it to the centre grid and moving it all over the place but this still keeps happening- any ideas?

I then tried resetting the settings in the mirror box but it duplicates inside itself on the default setting- mirroring on the other axes makes it upside down or back to front or miles away. I am in subds btw and I have tried freezing transformations etc. I really don't want to start again- is there a way round this? If I convert to polys it mirrors fine but I really wanted to be in subds for this stage.