I've recently picked up a cheap Citroen AX as a stopgap whilst the
family's main car is in for a major overhaul. I love the AX especially
the 1.5D fuel consumption (65mpg). Having looked underneath I've found
evidence that the drivers side rear wheel has been rubbing the side of
the wheel arch, the paint is worn away and the inside edge of the tyre
shows a little wear (you can still read all the embossed print, if
just looks like it's been rubbed).

The handbrake also barely works on this wheel, and the car sits a
little lower on the rear drivers side wheel than the passenger side.
The drivers-side wheel is covered by the wheel arch by about and inch
but the passenger side wheel is not obscured by the arch.  Whilst
stationary there is no noticable difference in the clearance between
the wheel and the bodywork on either side, but obviously I can't check
this whilst I'm moving.

Is this a common problem, and more to the point is it likely to be
expensive to fix?

Regards,
Jason.