Many thanks to all who provided advice on replacing my seized ball valve. The offending valve gave up fairly easily and in the end I did not need to apply heat. The through hull fitting was in good nick and not replaced. Thats the good news ! Having removed the old valve I decided to investigate further with the assistance of the boatyard vice and a large spanner. The shell of the valve can be unscrewed by applying heat and lots of force with the spanner. The threads that screwed into the other half of the valve were so badly eaten away that over 50% of the circumference just came away, a disaster waiting to happen. There is no sign of pinking (dezincification) just a dull rusty surface where the threaded flange had cracked away some time ago. This valve was the silver bodied variety seen in chandelers and was fitted as original equipment 7 years ago. I am now checking the 9 other similar valves with a more critical eye but they show no tell tale signs like the valve just replaced. A one off or should all these valves be replaced ?