Steering head locknut isn't reverse threaded is it?  I've tried a spanner and whanging on that with a hammer, as well as the srewdriver/hammer, can't get it to budge,
The steering head is beyond notchy, the ball bearings must be powder in there.

It is NOT reverse or left hand threads.

Well, there goes that hope. It's extraordinarily stubborn.

Did you loosen the pinch bolt first?

try some heat?

I'll say this before RT325 does - get a bigger hammer!

That worked  I

At this time, I must qualify my statement that the threads are not left hand.
Why? Because I ASSUMED that this was a standard enduro that is in question. Since there is no mention of the brand, model or year, it is possible that some other manufacturer uses left hand threads.

If you are lucky enough to have an enduro that the top triple clamp is removable, you could heat the nut, then spray it with a penetrating lube (Aerokroil). You may even find a socket and long flex handle that would engage several of the teeth and apply uniform force as you loosen it.

Bigger hammer got it DEET,  US DT3. It just took more force than I was really comfortable with.

 Ha, i was going to second the pinch bolt theory around that. But doesn't squash it like it does to forks undoing that top big nut. Can be stubborn though. Think mine is the crank bolt out of my Honda Acura Vigor motor with a high top 17 or 19 hex. Pic coming but totally irrelevant to your problem except it's just things i did when ran out of parts &, oh, that'll fit.