Xantia Aux belt tensioner bolts

[magictorch]

Evening all.
On attempting to change my aux belt on the 1.9TD Xantia (series one) I managed to round off the top bolt hex. I will be using some Irwin bolt grip socket to remove it tomorrow. Does anyone know the specification of the upper and lower bolt?

On service citroen the bolt are shown as being replaced with a self tapping screw and anti vibration washer. Not an expensive replacement from the main dealer, but curiosity has the better of me.

Edit: Manual tensioner