For the amusement of the curious we finally, yes FINALLY !!! got this head unstuck last week after all this time, using a hairdryer on the highest setting to warm up the base of the head, a plumbers adjustable spanner with quite sharp 'teeth' and one person to push down on the spanner while the other turned it. Due to the very narrow and bevelled base of the head, it had proved very hard to grip with anything else. It has done very minor damage to the panorama scale as the teeth bit into the metal slightly but it is very minor and I don't care as I can now get a different head, one with a base that is more grippable or of a design that allows the ball to be tightened enough to turn the head without damaging the ball-grip system / causing the ball just to slip.
Most pro tripods have grub screws under the head base . Not many people use them. Some of them only prevent the head unscrewing. others also prevent the head screwing on too tightly. Using them can prevent this happening again. It is quite easy to drill a location point in the first sort to lock the head screwing in either direction.
Thank you so much for pointing this out. With the design of the Benbo head, its not going to be possible to drill up through it. I will use a head with a much wider base next time, the one I used was small so was well inside the diameter of the benbo mounting area, again making it hard to grip to unscrew. Its a great tripod, I just need to choose heads with more consideration.
I also now have another tripod with the grub screws you mentioned and they are excellent at stopping the head twisting, esp when the centre column has been moved to the horizontal position (this position with a heavy camera and lens is what caused the over tightening in the first place). Thanks to your tip I will avoid anything without the additional under platform securing screws in future as its such an improvement in securing the camera, so it does not unscrew itself from the tripod mounting plate/screw or get too tight. I am guessing this situation is less of a problem if you only use the centre column in the standard vertical position.
I don't like being defeated!!
Well, I gave up for months then thought I would have another go. Its been annoying the hell out of me all that time. It was my last try before considering getting out the hacksaw and wrecking the head to get the tripod free.
I am ridiculously happy that I finally defeated the stuckness
Thanks everyone for all the suggestions. I hope you find it helpful / amusing to be updated.