Open the top of the fuel tank and have a look at the sender unit trying taking out leave the connection on and try moving the arm manually, if your fuel gauge reads full then the sender is getting stuck. could do with moving up and down manually to free it up. if the gauge is still reading the same then you will need a new sender unit.
before doing replacing that though use vag com and test the instrument cluster and see if the needle is moving all the way up to full could be a damaged fuel gauge.
if you have a saloon it will most likely be under the boot matt at the back of the boot on the right hand side normally held down with 3 screws. if its n advant then it could be under the seats or in the boot im not to sure on that one never owned a advant yet.
i would recommend running it low as possible before opening the tank and make sure you have as much ventilation as passable as the smell of fuel will stick for a while.
when you take the access panel off it will be held down with normally a screw type ring these can be exstremly tight if its not been off yet. if you crab a long screw driver flat head with a blunt end and a hammer and gently tap it round it should work its way off with ease.