Can't Undo Steering Stem Nut

[2-stroke junkie]

Hi
I'm trying to get the steering stem nut undone on my XR400 (2003 model); so that I can get the upper fork bridge off; so that I can get to the screws to take the steering lock off, so that I can get a new key cut for it (when I bought the bike a year back the dealership denied any knowledge of a key for the lock).
I have taken the handlebars off and pushed them forward, out of the way and I have undone the clamp bolts of the upper fork bridge. (The front wheel is also off, for trueing).
My Clymer's workshop manual doesn't mention anything other than 'remove the steering stem nut and washer'.
I have tried my rachet and t-wrench/spanner; and I have tried spraying it with lube, then tapping it with a hammer to loosen things; but the nut will not even budge a little bit.  ???

I'd appreciate any useful advice 

 

[Sprocketbek]

 

[Jughead]

 

[wolf skaap]

 

[spankme]

 

[lecap]

 

[GRIM]

 

[2-stroke junkie]

 

[vigilant]

 

[wolf skaap]

 

[lecap]

 

[sidetrack]

 

[2-stroke junkie]