See my edited post, I meant to say if you unscrew the plate - basically there is a ring, you knock the ring round and that will release the sender, you lift it out and there is the sender mechanism.
Basically what you need to do it alter the arm to suit the mis-read - so if it reads much lower fuel level than what is really in the tank then you need the float to be bent down a little so that as the float drops the arm doesn't sweep down as low, thus you will end up with a higher fuel reading - other way if it reads too full that means that the arm float is sitting too low, needing bent up a little, so even with say 3/4 tank of fuel, the arm sits higher i.e. in the fuller position before it should because the float is sitting down in the fuel.
Small moves, when I did it when I fitted my new tank I got my tank low, like just enough to cover the tank but still enough the pickup could suck fuel, so maybe 2-3 litres in the tank, then I calibrated it so that the fuel light was on and it was showing empty, then add about 6 litres and make sure that the fuel light is still on and still showing near enough empty/low, then get it so for the sake of an extra 3-4 litres it turns off the warning light, you want to get warned when you have about 5-6 litres of usable fuel really - maybe even more if you want - I think I get a warning when I have about 45miles range (probably more but I never try it), my engine gives me about 28mpg so about 7.28litres in the tank.
It is better to read lower than it is than fuller and conk out. The tanks are small and you don't get far on a tank really.
