This is a job most Capri owners will have to do at some point in their cars life as they seem to be rather unreliable. For those of you who don't know, this is the gadget that fits in the side of the fuel tank and does two things: it is where the fuel is pumped from to get to the engine and it is the device that measures the height of the fuel in the tank and records the information on the petrol gauge.
To access the sender unit, take off the rear passenger wheel and you will see it on the side of the petrol tank.
Disconnect the two wires on the sender and short them together. This should make the petrol gauge read full. If it doesn't then the gauge itself is bad and needs replacing. To replace the sender you will obviously need to wait until the tank is almost empty but I would still have a clean bucket ready just in case. Make sure you have purchased the correct sender unit; some cars such as mine have a send and return fuel pipe which requires the genuine Ford part which costs about £28. Check that the gasket is included; I seem to remember that Ford sell it as a separate part number.
Note that since writing this several years ago it has become impossible to buy a brand new fuel sender with a send and return pipe. They are like hens teeth and only occasionally appear on E-bay. However you can buy a single pipe version and I am sure that it would be easy enough to tap a bulkhead pipe union through the metal. It really is just a return hole for the fuel.
Remember that working on petrol tanks can be very dangerous and to avoid the risk of an explosion there should be no contact with metal objects. The sender is removed by rotating it anti-clockwise like a jam jar lid. To do this, I used a flat edged piece of wood that fitted in one of the notches of the sender and yapped it gently with a hammer. Keep moving the piece of wood around until it starts to move. You need to turn it 90° to get it off. Eventually you will be able to remove the sender and float system. You will observe that the end of the pipe has a small gauze filter on it. It may be necessary to put this on the new one but take the time to clean it first in some petrol. As they say in the Haynes manual, fitting the new one is the same procedure in reverse, but remember to fit the new gasket to the sender before tightening it down. When in place, the float assembly should be pointing down. Finally fit the fuel pipe (or pipes) to the new sender making sure they are the correct way round and re-attach the wires to the terminal lugs. This is harder than it may seem as they really need soldering (which will require a large soldering iron) or as I did use some crimp connectors that push on over the lugs. Once finished, I covered everywhere in silicone sealant to stop water getting to the contacts.