Not pretty but relatively straight forward.
You need a new sender unit, readily and cheaply available for a 200Tdi Discovery, 2 new 8mm sealing olives and if you're feeling flush then really go to town and get two new pipe unions to replace the old ones, mine were grotty but would go again.
New sender unit, it comes bare with no seals or sealing olives so be warned!
The old sender with porous pipes.
Ooops, but yeah it was expected.
Now then, remember where I told you that the new sender came bare, well this is the part where you need to get yourself 2 new sealing olives as your's will be the same as mine and they'll be rusted solid.
I didn't have days to wait for new ones to come via mail order and my local hardware stores didn't have the olives that small on the shelf so I had little choice if I wanted to get it up and running quick smart but to buy an 8mm in line coupling just for the olives.
Turned out just fine to be honest and just as cheap as buying a pack with more than I needed that would go to waste as I only needed 2 olives.
Just undo the small wiring plug and pop the earth wire off the sender.
Undo the big plastic nut and as you pull the old sender out the rubber seal will do it's best to swan dive into the tank just to piss you off so be prepared to grab it and pull it out with the old sender.
Thanks for looking.