Yes, lots
1) you need to do something about this. The fuel tank is effectively in the cabin.
2) it could be the breather tubes, it could be the tank, it could be connections to the tank ie to the filler cap and to the engine, it could be (rather less likely) the fuel gauge or the reserve unit (I’m presuming the 406 has this)
The following suggestions rely on Bristols conservatism, my experience is with a 410
3) to investigate I’d recommend first removing both front and rear seats to give you room to manoeuvre.
4) behind the rear seat backrest you’ll find a flat panel, possibly covered with some, by now very tatty, sound insulation. Remove this panel (the later cars used self tap screws, I’m not sure about the 406) to expose the fuel tank.
5) with luck you’ll find no obvious leaks to the tank itself but you won’t be able to get to look at much of it unless you take it out
6) If you’re happy that the tank is ok I’d recommend checking the outlet to the carb, which on a 410 is taken from the top of the tank via the reserve unit, check it’s gasket as well. Ditto check the gasket on the fuel gauge sender
7) I’d also recommend changing out all the breather pipes as well as the filler pipe as a matter of course, even if they look ok
8) If you decide that the tank has got to come out first get underneath the car, find the drain plug and empty it. It’s heavy enough as it is. Then it’s a case of getting back in the car, undoing and removing the very obvious straps that hold it place, and wrestling it out of the car. It’s heavy and awkward and very possibly well stuck down and you’ll be damn glad you took the front seats out. A friend might be very handy!
9) If there are leaks send it away to specialists to be fixed, (I was very strongly recommended by none other than Brian Marelli NOT to go down the diy route with any of the tank sealants you might perhaps be tempted to buy and mess about with)
10) I forget which specialist Brian recommended me to but these people appear to have the right credentials.
https://radiator-repair.co.uk/repair...op/fuel-tanks/
Hope this is some help