Un-happy wanderer - Steering / tracking question

[Fizpop]

Dear all - looking for the opinion / reassurance from the PH massive.
2006 Fiesta 1.4 TDCI, was begun to wander on uneven road surfaces / trying to tramline on the road contours. It brakes in a straight line, no steering knocks and feels fine on smooth roads etc.
I was thinking a tracking or alignment problem, or perhaps some uneven tyre wear contributing to it.

Any other suggestions? I'll probably get it looked at when it's in for its next service but if I can get the consensus of you guy's it'd be appreciated and possibly put my mind at rest.
Many thanks