Vauxhall Corsa c 2003 alternator?

Hi all i need to replace the alternator on my daughter's Corsa. I have removed the engine mount bolts to allow a little more access .the first problem I've found is I can't seem to get the tensioner to move to slacken the belt for removal by trying to turn clockwise nothing by trying to turn anticlockwise the bolt just seems to turn and that's it . Please any idiot proof help with diagrams or videos if possible greatly appreciated as YouTube searches for hours is of little help.