Noticed yesterday the front axle on my '91 disco tdi sits slightly towards the nearside, around 3/4 of an inch. The front shocks are also slightly offset, im wondering why this is...
could it be the radius arms have taken a bang at some point, and are slightly bent to one side? drives fine on the road.

What parts hold the axle in the center on the chassis?

thanks

The panhard rod centres the axle.
Have you got lifted suspension? That will push the axle to one side and you will need to fit an adjustable panhard rod to correct it.
Otherwise, check panhard rod and radius arms for straightness / bent mountings and all the associated bushes.