Now then Farmer G.....
A few little facts first.
1) John Deere made all the 40 series tractors in the Mannheim factory in Germany. They never had a factory in England. So your tractor is actually made in Germany.
2) The serial number you quote is actually the engine serial number. The tractor serial number is on a plate above the front axle on the right hand side! Incedentally the engine is made in the engine factory in Saran, France!.
So, on to your problem, plate on the side of the fuel tank?? do you mean the sender unit on the right hand side half way up?? Assuming you do, grip the notches of the outer flange with a pair of slip jointed pliers & turn it anticlockwise (after disconnecting the fuel hose & 2 wires) Replace the o-ring with a new one (L33601). Look very closely at the fuel pipe in the centre of the sender, they usually leak from there rather than the o-ring, if so you need a new sender (AL68201). See a John Deere dealer..
Tractortech., March 2010