Kicking myself, should've had it in 4 at most (and that's my first fail to boot); last 3 guesses were Birth, Mirth and Firth... however my second guess was Rigor, so I should've factored in there was a G but my brain just blanked it out for some reason until it flashed up as the answer. My disappointment (in myself) is immeasurable and my morning is ruined.