PCD100160 or GRC1184? Both seem to be spec'ed as suitable on Fleabay.
PCD100160 looks like a later part from its part number format (post '95?).
GRC1184:
GRC1184 - Mini rad cap mini twinpoint expansion cap(screw on)also metro (minispares.com)
PCD100160:
PCD100160 - Mini Radiator Cap | Mini Radiator | minisport.com Mini Sport
Dieter Koennecke discusses it in the MG(T)F space here:
MGF MGTF Coolant Expansion tank and cap (mgfcar.de)
"They just reworked the tooling and put a white dot on the correct made parts."
Has anyone any experience with the difference?