That would be the first thing that I'd have checked out.

Does the car "pull" one way or the other at all? thats another sign of the tracking/alignment being out.

Edit...

I presume that your from the uk lol, we all have the same complaint about road tax and road quality!!

It will be cheaper if you take it to a smaller garage than a main dealer. What age astra is it? I would reckon it would cost you the price of a new tyre, plus the labout to fir and align it. It would vary on the actual cost. Ring round, see what quotes you get. If you want, add me to your contacts and I could check with my garage i trust for a figure that would show you what price range to look for.

To test if it is pulling, find an empty straight area thats flat and gently set the car on a straight course, and don't steer, then see if it veers one way or the other. Do it slow tho and obviouslt dont let go of the wheel completely, just incase!!

could be alignment, but most of the time if it is the alignment you wouldn't ware one tire, could be the tire needs balanced, or if you have hit a curb or pot hole, could be out of tow or camber, alignment usually run around 40 to 50 dollars.

Scrap it, it sounds like one of those eyesores that blight our roads: Probably not roadworthy and hideously polluting.
DONT put any money into this banger and save up for a decent motor, young lady.