Thank you all for your replies, the links have been very helpful and we'll look at purchasing an overhaul manual. Just got to figure out which model the engine is as the data plate is missing, hopefully there may be something stamped on it somewhere.
Thanks to the links I've now identified the troublesome thing as the Thrust Bearing Nut, so it has a name rather than some of the other colourful descriptions it has been called recently...
As we have had no success in attempting to undo the nut in the traditional RH we may as well have a crack at applying some more robust force in the opposite direction, nothing to lose. The engine had been in the weather sitting on its back so rain would have collected around the prop shaft, no doubt there would be corrosion between the nut and shaft by now.
Again thank you to all who have responded, any other input or useful tidbits from those who may have had experience with these engines would be most welcome.