I do not think that Harriets birth was registered and I cannot find a registration for her sister Mary Ann either.
John Haylock married Ann Rankin in the Newmarket district March qu. 1857, volume 3b, page 758 (John is incorrectly listed as page 755 on FreeBMD). In 1861 they are living in Soham with Mary Ann age 1. In 1871 they are with Ann's father John Rankin in Broad Street with two daughters Mary Ann and Harriet. There is no evidence that they had lived outside the Newmarket district although it is possible, but there is no trace of a registration for either child. I suggest that you trace a copy of Soham Baptism Registers to see if the children were baptised in that parish.