Audi A4 SE (2003) Heavy Steering

I recently bought a second-hand Audi A4 SE (2003). The steering was fine at first, but now (after doing around 3000 miles) it is rather heavy and the resistance of the steering wheel is inconsistent,
especially at low speeds. This has made maneouvering the car around roundabouts and town streets very uncomfortable.
Another thing that I have noticed is that the steering wheel does not sit dead-centre when I want to go in a straight line. I'm pretty sure that it did when I first got the car.
Does anyone have any ideas what is going on and what the best thing would be to fix this? Has anyone experienced similar problems?
Thanks in advance,
Andrew
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