Mike, in his opening post Derramax said:
"I tested the new unit by moving the arm and watching the gauge and it worked like it should." so he knows the new sender is working and is within range.

if you remove the sender/ tank drained, you can put a mulitmeter on the leads and lift and lower the float. You'll see the resistance and know that is working.  A hole in the float can be epoxied. And if it has a hole in it, it will have gas in it when you remove it. Easy to see.

I did the Flashlight down the filler neck and honestly I can't tell what the float is doing. Can't see the darn thing. I'll try the hanger trick when I get home. In the pile of paper work I saw on one of the receipts that the tank was changed and I suppose I should run the number and see what comes up.

Even though you have replaced the sending unit, use a flashlight to look in the tank through the filler neck to confirm that the float is floating.  If the float is not on the top of the gas, the float arm could be bound up or the float could have a perforation allowing it to fill partly with gas.  If the float is not on top, drain the gas below the sending unit hole in the tank, remove the sending unit, and inspect it completely.

If the float is floating properly, turn the key to the run position but don't start the engine.  Use a stiff coat hanger or similar to lift the float and then push it down as far as possible.  Watch the gauge and pay attention to what you feel through the coat hanger.  When you lift the arm all the way to the top of the tank, what does the gauge do?  Remember you will have to hold it up for a minute so the gauge reading stabilizes.  Now push the float all the way down and make the same observations.  Obviously the float arm should be able to move without mechanical binding and the gauge should read Full, then Empty.

You have found your car is not what you expected.  How long have you owned it and how long has it been in the U.S.?  There is a slim chance a previous owner installed a late model gas tank.  The late tanks have a different internal structure and require a sending unit with a Z-bend in the float arm.  If you install an early sender in a late model tank it may bind up and not move freely.

welcome to the 77 mini club !!! mine is a 1977 leyland classic mini from Belgium, i have a very very clean title under my name.  I also have the previous titles(im the 3rd owner btw) that has many holes in it,they were old and moth eaten. i put it inside a plastic and preserve it. and somewhere in the middle the name "ringo" appears, it's pretty unclear because the color and the paper is deteriorating, but i really suspect this car was owned by a guy named ringo, from what i heard he was a member of a band called beatles.(im too young to know,and idc) but anyway, i love my mini, i pay $75 /year for the insurance and 30buck for registration, and it has given me many happy miles and smiles.

and also, when i got my mini, the gas gauge was not working either, i took the sending unit thingy and clean it really good, made a good negative contact and now its working perfectly, when it's empty the needle is really down the last line, i know it because i always put 2bucks everytime i go out for a leisure ride., lasts me 55miles.

Well turns out my 1970 mini is actually a rebadged 77. Doesn't really bother me outside of it calling into question some of the history paperwork from 3 owners ago that came with the car. Also my insurance values the car at what it cost in 1970 and not 77 so its a negative there as well. Anyways, I've done a lot of searching and haven't really found an answer to my fuel gauge problem. When I got the car the gauge always registered F and it never moved. My temp gauge works fine so I determined it was a sending unit problem. I tested the new unit by moving the arm and watching the gauge and it worked like it should. Installed and filled the tank and found that the gauge now reads 1/2 full when full and stays there until there is about 1.5 G left in the tank where it falls to somewhere between 1/4 and E. At least now I know when I am running low on fuel. I know gauges in a Mini are not the most accurate but I think it should be better than this. I read somewhere that the sending unit can be fine tuned by removing a screw in the unit and twisting something? Could it be that the arm is not going all the way up in the tank and needs to be slightly bent or the float not floating? I have to say its nice to be on a forum where people are genuinely interested in helping others and wont ignore you if your question isn't about the latest hack move to lower my car to undriveable levels.