Fuel Gauge Problem Ford Ka X - 2001?

The fuel guage on my Ford Ka keeps moving back and forth and it is difficult to tell how much pertrol i actually have in my tank. Often 3 days after filling my tank it's showing as being empty, previously after filling up it would last me up to 2 weeks (dont do huge amount of miles in it). Didn't pay much for the car - afraid to take it to garage incase it costs alot to have it fixed - can anyone advise me of the possible cause?

Thanks