I owned a 1984 GB 36 with a British Leyland. Mine was the 6TX200 which I detuned to a 165 by removing the turbo intercooler. In 15 years of ownership the only repair I ever had to make was to replace the fresh water cooling pump (not the sea water pump). The engine is bulletproof. I was hesitant to buy the boat because I was not familiar with the engine but it proved to be a fabulous power plant. I never did anything more than change the oil, filters and belts. I sold the boat to a Canadian who moved it from Stuart FL to Ontario on its own bottom with not so much as a hiccup. He sold it and I met the new owner who has used it for two more years with no issues.
FYI, American Diesel in Kilmarnock VA was the north american distributor of the Leyland. They have a full stock of parts. Brian Smith at AD is an expert on the Leyland. Give him a call and he can sing its praises to you.
Magic, 1996 Grand Banks Europa
Westport, CT and Stuart, FL