The plastic is delicate as Alan says but should be okay if left alone. I had a fitting snap on the top of mine due to a ham fisted garage. Weirdly it didn’t really show up until I brimmed the tank and it poured out the top!
If nothing is cracked on the fuel lines to the sender and it really is the big gasket on the fuel tank sender, I would just take it loosen the big clamp but thing, clean and lubricate the seam with Vaseline and put it back together, as an alternative to replacement. Doing it this, cleaning in-situ, means less disruption to the sender and pump. The fuel unions are just set in plastic so can snap and are best left alone unless you really do need to disconnect them.
You can access this from inside the car, under the rear arm rest— you probably know this . Be careful when you are working on the sender not to damage the the big float arm if you do decide to be brave and remove the sender from the car fully. Also if the fuel pump pickup filter falls off when you are moving the sender about, don’t ignore it as your fuel pump will likely fail soon after without it.
Typing “Volvo fuel sender gasket” or “Renault fuel sender gasket” in to eBay shows some suspiciously similar looking seals if you really do need a new one.
Correct torque for the big retaining but is 50 Nm, but as you will likely be using a hammer and a drift, this is all guess work to get to “very tight”. In an ideal world, two people tapping it tight 180 degrees part would probably be best.
P.S. Most obvious place for an actual leak is the fuel filter as it gets butchered over the years with various “highly skilled” mechanics having a go at changing it.
P.P.S. What about the fuel tank breather? If it blocked, the fumes are going to try to get out somewhere! Just a guess though. Also, someone else had a snapped fuel tank breather here:
viewtopic.php?f=24&t=33567&p=199090&hil ... er#p199090
There are theee different designs depending upon age
