Undo the two screws and 1 nut that hold the Yellow, Red and light Brown wires on.  No need to worry about the Blue one.

Undo the two 14 mm bolts that were under the Yellow and Brown wires. A bolt should drop out the other end allowing you to slide out the tube.

After removing both tubes, the bottom locator will just slide off

Unscrew the centre tube from the top housing.

Slide the outer tube off, then unscrew the exposed inner tube - I sat the outer tube back on just for the picture to show there are two tubes that remove to expose a centre probe.

After removing the two outer tubes, undo the nut (above right) and remove the central probe.

Soak part of the clean rag in methylated spirits and wipe all the tubes and the probe.   Methylated Spirits mixes with petrol so it won't contaminate the fuel.

Now comes the important bit. Inside the smaller centre tube near the bottom (left hand side in above pics) is a locating piece for the probe.  

Gently insert the probe into that end of the tube and push to slide the locator towards the other end about 1 cm (1/2").   This will ensure the locator catches the end of the probe as you reassemble, holding the end of the probe firmly in place and not letting it touch the sides causing the fuel gauge reading to jump about.

Reassemble it all and fit it back into the tank.   It's a bit fiddly to get back in properly, the bottom tends to catch inside the tank, but persist and be gentle and it will go back in correctly.  Replace and tighten the bolts, reconnect the plugs and the job is all done!