Paul has kindly provide step-by-step instructions on how to install and replace the fuel tank "O" ring. Thank you Paul for providing these details and being available for other members to contact you in case they get into trouble or need some general advice.
Before you begin the replacement of the fuel tank filler O ring first consider this -
I think I/we may have over looked other aspects affecting this problem. My problem was fuel vapour in the cabin when the tank was filled more the ½ full. When I did the procedure below 2 things struck me, was the cover over the tank and the breather sealed? (in my case no). I assume it has not been touched since manufacture, certainly the split that allows the breather pipe through has never been sealed.
When fuel is agitated in a tank, vapour is generated and allowed to vent via the vent pipe. The vent needs to adequately vent to atmosphere. Venting or leaking into the car interior is not permissible. My observations are;
Vent pipe is too short. It ends 30mm below the boot floor section once it leaves the boot space.
The Vent pipe and filler neck boot cover are probably at best partly sealed.
I have extended my vent pipe with an addition of a plastic pipe to the offside of the car and I have sealed the vent and filler neck pipe cover with silicone. As well as replacing the filler neck to fuel tank O ring.
I have driven and tested swerving from side to side on the road to exaggerate fuel sloshing in the tank and generating fuel vapour. After a period leaving the car to stand with windows closed there was no fuel vapour apparent in the car cabin.
I did drive the car after replacing the fuel tank O ring and without sealing the filler and vent pipe cover with it open. There was vapour apparent in the cabin.
My only conclusion is that you should check the fuel filler neck cover for seal or reseal before you replace the filler to tank O ring.
Now the steps...
First take everything out of the boot. Check your spare for pressure, mine was flat!
The new and old O ring. The old O ring (the one on the right is the old one and doesn't look too bad)
Then remove the five Phillips head screws from the plate on top of the fuel tank that covers the fuel tank filler tube.
Remove the breather line on the inside of the filler tube which is located inside the wing. Pull off the rubber connecting hose.
Then persuade the filler to move inwards such that the filler tube just enters the boot.
At this point I bought out the vacuum cleaner. Fearing dislodged dirt may fall in the tank, as the filler tube is removed. I laid it close to the tube and left running. Helps reduce the fumes once the tube is out. Use a type 3 face mask respirator if you are allergic or don’t want to breathe fuel vapour for health reasons.
Next rotate and pull out the filler tube (again a bit of a fiddle but persist and it will come)
The O ring will remain in its groove in the tank neck
With a small screw driver carefully remove the tank O ring.
Clean and inspect the tube filler neck where it will enter the O ring. I rotary wire brushed mine then fine emery papered some rougher bits smooth.
Note: I used Loctite 457 in the absence of 541 as Tom advised both are anaerobic i.e. go off in the absence of air.
Push tube into tank and O ring. Pushes in quite easily.
Refit tube, breather and tank cover. Don’t forget the breather tube goes in on the far side of the tank cover and has to go on/in with the cover. Otherwise it won’t fit again a bit of a fiddle but persist it will go.
Test by filling. I did by progressively filling ie ½ then ¾ then full. If it leaks at ½ or ¾ then there is less fuel to remove to rectify the problem
Big thanks to Tom for supplying the new O ring.
Finished job. Boot scrubbed out washed flushed and dried. All tidy. Just need my spare rim powder coated and all the other stuff can go back in.