Well basically it is all about the costs.....
I don't think you really understand the costs.
i am spending £120 a month on petrol to start with
No, you are not. Either you are not spending £120 or you are doing much more than 50 miles a week.
What i wonder is if i got another car with a better mpg
Smaller engine etc
And did the same amount of miles
Would it cost less on petrol?
Would the insurance be less?
Would the road tax be less?
Yes to most of these but of the biggest issue is of course how much less. If you really only do 50 miles a week the savings will be very small, so small that it simply isn't worth the bother.
Of yes two of those then it is worth it for sure
How can you possibly say that without knowing the numbers?
Lets assume your car does 22mpg around town. A reasonable worst case scenario. It's probably better than this, it's almost certainly not worse.
Lets assume you consider replacing it with a crap car that does 40mpg around town.
50 miles a week, 200 miles a month, 2400 miles a year.
Fuel cost with your Audi: £46 a month
Fuel cost with crap little eco car: £25 a month
Why bother for such a pathetic saving?
Either your maths are terrible, you do far more miles than you've told us or your car is broken.
If your car is broken, fix it, because you'll need to fix it anyway if you want to sell it.
With mileage as low as yours the biggest cost of motoring will be unscheduled repair bills. Therefore the only credible saving you can have by changing to another car is if your Audi is very unreliable and constantly costs you money to fix it. If it doesn't, then don't bother.