I've had a similar thing where my RR wanted to "stick" in the going round a corner position. One of the steering UJ joints was cream- crackered. I think they're only about a tenner each or so.
Easiest way to find out whats causing it is to seperate the steering column from the box via one of the UJs and then working the system a bit at a time to work out which bit is stiff ie. either the steering wheel end or in the box / swivels. You can further the search by spliting the ball joint under the steering box to isolate the box and the swivels / joints from each other.
Shouldnt take too long to isolate the problem but it might take a bit longer to fix it.
Dave