My fuel gauge shows full when full but only drops to 3/4 full when the tank is empty. Is there any way I can change this with a resistor or somthing or is it just a case of buying a compatable fuel sender.

Compatible sender I'm afraid....

^^^ wot he said unless you are able to design and make an electronic circuit that can interface between the two.

the float is definitely at mimimum position when you gauge shows 3/4?

In theroy it is possible to add a resistor in parrallel to get the range. First off make sure your sender is not sticking. Measure full resistance , empty resistance using a multimeter. then tell me what the gauge requires, i`ll see if i can sort out a resistor network that will work.
If you do not know the range the gauge requires, buy a 10, 50, 100. 220, 330, 470, 1000 ohm resistor from maplin (all that will cost about £1), connect each to gauge and give an accurate reading as possible for each value.
Darren