I have an oil leak from reduction box on lister engine?

i have lister sr3 marine diesel air cooled engine on my boat with gearbox and reduction box and it is leaking oil from the reduction box, it is coming from where the prop shaft goes into the reduction box , now is there some kind of seal or bearing that has gone , i cant seem to get hold of lister.co , where can i get a new seal if it is that or will i need another reduction box fitted, many thanks