Citroen Zara Picasso?

I have a problem with the gauge that shows how many miles I have in the fuel tank. I have had a new sender unit installed but it has not helped at all. When I start the car in a morning the gauge may read say 435 miles (after I have filled the car with fuel.) By the time I have backed out of my driveway the gauge has dropped to 199miles. As I drive forward the gauge still continues to drop to about 165 miles. After I have driven about 14 miles the gauge will now show something in the low 300's. This does not happen just because I have filled the tank. I can drive it in reverse for about 10 yards and the gauge will show a loss of about 20 miles. Continue driving backwards and forwards and the gauge just keeps dropping and dropping. I have had it into a Citroen garage and they cannot find a fault. They have been out on a test drive and I have shown them the problem but they do not seem to be able to come up with an answer. Can anyone out there shed more light on this problem and come up with an answer.