I live in Sydney, Australia and am also related to the Taplins of Stevenage, Hertfordshire.Joseph Taplin had 3 brothers, William, John and Thomas and they were all Christened in Stevenage on the 1/2/1861, and a sister, Emma who was Christened on the 4/8/1850.I am descended from Thomas Taplin who married Eliza Knott in Stevenage in 1876. My father was a Taplin and he migrated to Australia in the 1950s when he joined the Australian Navy from the Royal Navy.
All that I know about the family is that William was born approx. 1832 and Sarah Allen, born approx 1831, to James Allen. She remarried when she was 42, to a Daniel Knott who was 56, on the 22/9/1873. Look up the 1881 Census under Sarah Taplin marriages 1873. I do not know what happened to William though. Unfortunately I havent yet been able to prove where William or Sarah where born as there are so many of them. I have followed up further on Daniel Knott though as he was the father of Thomas's wife Eliza.
Would love to talk to you further or hear from anyone who can help us out.