I have a problem with the steering on my Aust. spec 91 mk2. The steering is not centering and is tending to settle at about 15 degrees to the left or right. Straight ahead can only be held by force. There is also a new dragging sound at full lock in either direction. There has been no previous steering issues and this has started with no warning.
I am going to lift the car to investigate and could use any advice on what to check for to try to get an idea of what is going on before heading for professional help.

Did you hit anything, kerb/pothole etc?  
 
Check the obvious, loose wheel nuts, bent struts/suspension arms, tyre pressures, steering ball joints, steering rack mounting, suspension joints, tightness of all suspension mounting points, alignment adjustment bolts etc.
 
You might also be able to see one (or more) of the wheels being out of alignement if its quite bad.
 
Also check wheel bearings.
 
Cheers

It could also be a hub just to give you some ideas.

Sounds like something slipped or broke.
If you haven't already done it, park the car on a level surface with the wheels straight ahead. Stand a few meters or so out the front, and look to see if there is anything obvious, like excessive camber/toe etc.
 
When you raise the car, check all suspension parts that arty mentioned,  and a good idea to check the rear of the car as well.
 
I once hit a bad pothole in my sdub, then noticed the steering wheel spokes were not level. Turned out that the strut/hub bolts had slipped, causing bad negative camber. (that also caused bad toe out).
 
 

have you recently had a sway bar installed?? sounded like what happened to me after mine was put in and i hit a bump to hard and it span around which stuffed up my steering.
 
10 mins rotating it back to where it should of been and was pefect again.
 

Yep. As reddtarga said, I meant everywhere. Even fitting one rear wheel with different offset causes the car to veer in one direction, especially under power. This is what I experienced when I got a puncture in the back tyre and borrowed another wheel.

If yours is not self centring that would point to castor though would it not ?

If its such a significant issue it should be visually obvious.

Cheers
Cheers

Thanks all for the feedback
Taking a look in the light of day it was clear the anti-roll bar had rotated 90deg.
 

 
It was was pretty straight forward to disconnect the link and rotate the bar back. The question is how did this happen? the connecting collar to the coilover on one side was loose so I can see how that could have moved, I assume there was enough free play combined with suspension travel under turning or to allow the whole lot to move.
Everything else looked ok so I'm hoping that it  won't happen again.