I've got a 1982 300TD and had some radiator problems last year.
I replaced my thermostat and coolant last summer and when re-installing the small hose that goes to the coolant reservoir tank the plastic nipple broke off. I pulled the radiator, a pretty simple job, took it to a local radiator shop and they replaced the the top plastic piece, boiled out the radiator and had it back to me the next afternoon. Ran about $100 if I recall correctly. Sounds to me like your dealer wants to sell you a new radiator. The big question though, is why are you taking your car to a dealer instead of an independent shop? Don't think I've ever heard of anyone being happy with service by a dealer.
1999 Triumph Daytona 955I - my speed fix.
1982 300TD - Gone, but not forgotten.