One of the disadvantages of Diesel is running out of fuel. If you do, you have to disconnect a pipe and manually pump a button to suck fuel from the tank back up to the filter. Easy enough to do but smelly and inconvenient.
The secret to absolutely filling the tank is what we call brimming, that is filling slowly till the fluid is sitting at the end of the filler neck of the tank. Diesel foams a lot, so it can take up to 10 minutes to fill this way. Also not a good idea if it's hot weather and you're not going to drive a reasonable distance, as fuel can expand and there's nowhere for it to go except on the tarmac.
The tank size quoted allows for an air gap, for expansion etc, but some of us use that gap for fuel, I do.