Random Fuel Gauge

Hi all. Pls forgive me as I know there is lots about this already - I've just spent the last 30mins reading all the historical threads!! Like many other people I have a random fuel gauge - it mostly just sits on R regardless of how much fuel is actually in the tank!

I've changed the fuel sender unit in the petrol tank twice and still have the same problem - at 1st the gauge reads nothing. If you do the reset (disconnect plug at top of sender unit, ignition on/off, reconnect) then eventually it gives you a correct reading. Then after driving a couple of times you get nothing - the needle does not move at all and sits at R????

This leads me to believe its the actual gauge in the dashboard but all the other clocks dials lights etc are fine???? Therefore I have 2 questions

1 - if it is the fuel gauge how does one fix it??
2 - is it something else I'm missing completely??

Any help/ideas/advise would be appreciated