After new tank and Sender fuel gauge not working

Hi guys, I have a defender 110 300tdi on a 94 Plate, recently I replaced the fuel tank, fuel sender ( replaced tank and all fixtures and fittings), now I'm having major issues with the original fuel gauge, prior to the tank replacement, the fuel gauge worked with no problems, however, since the replacement, the fuel gauge won't work, there is a caveat, if I fit a fuel gauge (I bought a new one as thought the old one was knackered) it doesn't work, but here's the weird bit, if I fit another fuel gauge in addition to the first, the second gauge works and shows the correct amount of fuel.

I'm thinking maybe it's the sender (fitted in the side of the tank) on installation the sender (fuel Pickup) was fitted as is with no adjustments.

Any ideas why the piggy banked Gauge works but the initial gauge isn't?

For context, piggy banked means a fuel gauge is fitted in the normal way, and a second fuel gauges wires are fed from the 1st gauge with additional wires.

Did you replace the fuel level sender, or is it pick up and sender in one unit?

Is it the new gauge that is fitted and old one piggy? New gauge Duff is my first thought.

Lynall it's a pickup and sender in one unit and it's side mounted to the passengerside of the fuel tank..

Tottot, it's both gauges, last night I tested both gauges off the vehicle and both gauges work, also, I have been out again today, both the fitted gauge and sender/Pickup work, this was achieved by using a wire from the fuel gauge sensor wire to the sender centre wire as fitted.

In my experiments from last night, as fitted, the sender wires are:- the top connection is Battery Negative, the centre wire is connected to the gauges sensor wire (right most spade connector on the gauge once fitted in the dash) the bottom connection (on the sender, which I believe is for the low fuel light is not fitted on my Defender.