Well I eventually got round to sorting this little problem out. i managed to siphon out nearly every drop of petrol into all my jerry cans, even having to make use of a brand new water butt for the last 30 litres
Photo 1 shows the plate on the top of the tank (already removed but back resting place for benefit of photo. That was a bu**er to remove!)
Photo 2 shows the hole in th top of the tank cleaned as best I could of old sealant.
Photo 3 shows the various sizes of screws and the old rubber gasket (obviously home made) from the fuel sender. Good old Prosport, why use one standard size of screw when 5 different types can be found in the bottom of your toolbox.
Photo 4 shows my nice new gaskets cut from neoprene reinforced cork.
Hopefully this new arrangement will work. A good dollop of Blue Universal gasket sealant was added for good measure to all surfaces. i also replaced the fuel suction pipe which was a piece of 9mm copper pipe which was slightly undersized for the olive and union. Had to buy a 10m roll of 10mm microbore form B&Q just for 2 feet of pipe. Nevermind, cheap at the price if it works.