I have used a very good quality hex key with a 3' pipe on it.
You could try hitting the center base of the hex recess with a center punch.
The hex bolt is screwed into the alloy arm off the motor, I dont know if it would help to heat that from above. Worth a try.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving