I had a fiddle around, and saved to master esm file. But not happy with the result. Can i reset to default, to resave?
Thanks in advance.

Perhaps you can explain what your talking about. The GECK cannot save master files.

The game master files are 'never' changed, the GECK makes plugin files for them, called ESP files.

I saved on the 'bethesda master data file' instead of a new one. I tried un and re-installing the game, but makes no differance.
But doesn't the geck have a 'reset to default', so you can undo changes?

Uhhh ... huh? You can't overwrite Fallout3.esm, unless you use 3rd party tools like FO3Edit or TESSnip. Any edits to Fallout3.esm are saved in an esp file.

If you have changes in your plugin file that you don't want, you can click on the 'Details' button when you have your mod selected. It shows everything your mod is changing. You click what you don't want and press the delete key. Anwser 'Yes' when asked if you want to ignore the change. Then load up the mod and save it.

Some things, if you delete them, will cause your mod to malfunction, so don't just delete everything unless you know what your doing. And make a backup in case you foobar it.