Disco Door Lock

The plastic ring that fits on the drivers side door lock and operates the locking mechanism has broken on my Discovery. LR parts tell me I cant buy the piece of plastic separately, I have to buy the complete barrel !! Does any one know if this part is available from somewhere or is perhaps the same as a lock fitted on other vehicles. The outside handles on the Disco door almost look like the ones fitted to B Leyland Marinas/Maestros. Any help or suggestion please? Thanks Roger