you dont mention any work done on the car that may be related, or how many miles its done. But chances are, with the age of these cars now, most bushes will be close to failure.
rear is only adjustable for toe, camber caster and toe on the front. Your local garage or tyre place wont have facilitys to set the omega correctly, no matter what they tell you.
Full geometric set up needed. However for this to be fully successfull you need good bushes front(wishbone bushes) and back(donut bushes). Check springs for breakages and steering idler for wear/play, and if your happy the front shocks are ok and not leaking get it set up.
You can just set it up without checking of course, but if the shocks need changing at the next mot, or any other related components need replacing, it really needs setting up again once replaced. So best to check first, set up once....
Have a search re set up and bushes. Well covered on here. There are also guides in maintenance for replacment. Hth