Ok, I have searched tinternet for the answer and no joy. So can someone tell me what BTN means.

It's just the label for a particular fuse in the fusebox.

It's still just a fuse

http://www.askanowner.nl/qa/view.asp?guid=&sid=2&qid=41642

The BTN fuse is the "Back to Normal" fuse. What this fuse does is resets the ECU (electronic control unit)... when you put the fuse in the BTN slot, it completes the circuit to reset the ECU.. when you remove it.. it's reset. This is not a fuse that should be left in.. only use it to reset your ECU if you're having problems. Hope this helps!