There certainly are a lot of jewelers/watchmakers named HETTICH!
Some that I could see (though none specifically at Hatton Gardden):
Clement HETTICH 199 1/2 Upper Whitecross St (Pigots 1823-4)
Karl HETTICH (jeweler) High St, St Clements (1860s-1870s - various newspaper items etc, died 1874)
F X HETTICH (watch and clock maker) Kentish Town Rd (partnership dissolved with J FALLER 1886)
W C V HETTICH (jeweler) at Lynn (died 1888)
HETTICH (jeweler) King St, St James (advertising for watch and clock repairer) 1913