This all started with a fuel leak which developed while I was overseas – I came back to find a large pool of petrol under the car, still dripping. It had not been run for 2 months!
On pulling things apart I found that the hose which leads from the pump to the filter had rotted through at the pump connection. Probably it was the original hose (32 years old) or someone had replaced with the wrong type.
I decided while I had it apart that I would check a few things so took out the sump (expansion?) tank, to find it contained half a cup of mud-like dirt, – not rust. The metal surface inside is in perfect condition.
So the dirt is coming in from the main tank, I think it is dirt from the environment or in the fuel.
I am now going to fit a low pressure fuel filter in the line from the tank to the sump, it should catch all future muck.
I have been told that all piping in petrol systems should be high pressure (fuel injector) type.
They are less susceptible to rotting than low pressure types.

Hi Paul, all the diagrams I have seen show 3 small pipes coming off the tank at the top rhs.
Mine has one pipe going to a little box, as I said.
My email address has been forwarded offline (Laurence)
A clear diagram of what this pipe does and where it goes will be most useful.
Thanks

It’s a V12 5.3l. I have found where most of the pipes go, and their purpose, except for the small connection and box on the rhs top of the tank. The box outlet line cars off towards the passenger compartment and has a disk shaped gadget in it, but I cannot reach it.
All of this is to try to remove petrol smells.