I have a Watermans 12 with a globe imprint on the length of the barrel and another pen which i believe is a Watermans 12 as that number is on the barrel end (and the section will fit on the first pen's barrel if interchanged) but otherwise that second pen has no other imprint on the BCHR.

Can anyone confirm whether Waterman did produce some 12s without any imprint on the length of the barrel and whether that is deemed an uncommon or rare pen? My one has no overlay but has one plain unhallmarked band on the barrel which i assume is gold plate.

Most likely, your pen with the mid-barrel band had its imprint on the cap. This was the norm when the barrel bands would have obscured the normal barrel imprint.
A regular, unimprinted cap will fit, of course, which is just as well considering that imprinted caps are relatively uncommon on their own.