Fireline

Hi all. Sorry, but I'm beating my head against a brick wall here. My temp gauge isn't working on the dash, and it appears that the sensor has died. (I was getting low readings for a while, then nothing.) My mk1 F has the original black sensor (YCB100370), which you needed the large washer for (MDY100040). The black sensor has been superceded by the blue sensor (YCB100420). It also says that I need to fit the small washer (MDY100080). What I can't see anywhere is whether I fit the small washer instead of the large one, or do I need to fit both? Can anyone please advise?