I think the Hartwig in my tree is probably not connected. My grandfather Alfred Hayman Hartwig was born in Manchester in 1860, went to Manchester Grammar School and worked for Jaffe and Co. where he was a director. In 1915 the surname was changed to Russell, his wife's maiden name, presumably because of anti-German feeling in the uk then. He retired to Llandudno and lived there until his death in 1944. My grandmother, Maude Hepburn Russell, died there in 1942.
His father was John David Hartwig and I havew a copy of the certificate of his marriage to Emily Makinson in the 1850s. JDH's father on the certificate is given as Joseph von der Lahr, and certain press cuttings refer to Alfred as Alfred Hartwig von der Lahr.
Frankfurt seems to be the town of origin, but I've had no luck in tracing them there.
Regards - Tony Russell