There are invariably things about our cars that irritate, for me a number of those are displays of ineptitude usually, but not exclusively by previous owners. The radiator filler screw was one such thing, it looked like it had been set upon firstly by false teeth, then the sort of Stiltson usually reserved for dump trucks.
The part number is PYP100500 & on eBay, they are £9-25 which is simply too much for something which is plastic & has the propensity to cross thread. However for a more reasonable £4-30 you can have a M12 x 1.25mm Steel Drain Plug Blanking with Recessed Rubber Sealing Washer, again from eBay supplied by mcconnectors (213). It looks nicer also. QED