Steering 'wobble' felt through wheel?

[31satara]

1998 A4 Avant Tdi 225k miles (The car does around 6000 miles a year, non-motorway)
When driving straight the steering wheel wobbles a bit - turning a little each way back and forth - it doesn't cause the car to change direction. When steering it can have a bit of a 'notchy' feel to it, getting slightly lighter and heavier. (It doesn't particularly pull to one side or the other, seems to depend on road camber. No noise on either full lock). Over the last few months I've noticed a gradual increase in this, in the last week more so.
Can anyone give me any clues as to where to, and what to start investigating?
(I did swap the wheels a couple of months back which didn't really have any noticable effect. There didn't appear to be any noticable play in the steering or wheel bearings. I'd also checked wheel bolts were tight!)
Thanks

[MrJoshua]

Check for play in the control arm bushes.

[31satara]

Thanks for the suggestion.
I've just jacked up each side and put a bar under the wheel and had someone lift it (like I've seen them at the MOT...) - no obvious movement in any of the control arm ball joints (upper and rear). Had a big screw driver around the bushings. Overall they seemed fine, although one of the upper rear ones didn't always sit exactly central and could be moved 5mm or so side to side (the rubber did also have little cracks on). The other upper rear had a little less movement. They were replaced a couple of years ago (with cheap ones).
Could this be enough to cause the effect described above or do I need to look elsewhere?

[31satara]

Solved it. It turned out to be the front tyres (one more so). Only very slightly out - when watched spinning I could just see a slight irregularity. Suprised it had such an effect - as soon as I swapped the back tyres on to the front all was fine again.
Two new tyres - much easier then playing around with suspension!