I have a Eberspacher diesel heater glow plug from you 251384010002 , it's come with what looks to me like a cork Washer at the top, do I fit it with this in place? I dont remember seeing one on the one I took out of the heater. Thanks

This washer is a fibre insulator washer. It is typically required. It insulates the two cables that connect to the glowplug from each other, to prevent a short circuit. There are however some early Eberspacher heater models that have a pre-prepared insulated 2 x cable connector that fits onto this glowplug as an 'all-in-one' If you have this connector type, then you do NOT require to fit the fibre washer

Please see images attached