Haynes Forums - Fuel gauge gremlin

I tinkered with the gauges today and in particular with the fuel gauge to see if everything works OK with the sender unit I have.

For starters, the gauges I have are VDO's Gold Heritage series (older analogue model) and the fuel gauge have three lugs marked "S - ground - positive".
The sender is from a jet-ski fuel tank and the Ohm range within the numbers as specified by VDO and it has two lugs marked "S - ground"

I connected the positive (12V) to the gauge, the S on sender to S on gauge and connected the two grounds to the sender and gauge. The gauge performed beautiful with exact readings as per movement of the sender float BUT wrong way around - sender shows full, gauge reads empty and vice verso. :?:

I the changes the two wires on the sender to try and reverse the reading but it stayed the same. Then I tried to change the polarity, IOW negative to the gauge and positive as ground and no reading then.
What is wrong and how can I rectify the problem without having to buy a VDO sender unit? I tried to fit the float upside down but the stiff wiring of the sender is to short for that stunt and this is when I swopped the wiring at the sender that should do the same trick but as said, did not change the readings.

Can anyone help?