During the clearance work I found another .303 cartridge. The headstamp was pretty dirty and or corroded, and in my ignorance, I tried cleaning it with a brillo pad and some meths.
The last picture reveals the red paint has just about all been removed, however both the writing and the firing pin indentation are very much visible. Better conservation techniques may have enabled all its features to be revealed and conserved.
The headstamp revealed the lettering:
B /|\ E 1942 G II
This identifies it as the cartridge of a .303 tracer bullet made in 1942, in the Royal Ordnance Factory the Royal Ordnance Factory in Blackpole (not Blackpool), in Worcester. The red paint (before it got removed!) confirms it was a tracer bullet.
The oval shaped indent in the middle was from a firing pin of the same shape – which means it was fired from a bren light machine gun. Every half dozen or so bullets was a tracer to help keep the gunner on target.