The problem is probably being caused by a heavily worn driver's seat. Consider buying a foam-filled comfort cushion with an anti-slip backing and adjustable straps that will secure it to the seat. Halford's sell them for around £30.

If you are considering replacing the car, then something like the Toyota Yaris might be better for your needs: www.honestjohn.co.uk/carbycar/toyota/yaris-and-yar.../