Hi Steve
I too have Merrills from Hunts in my tree (my mother!) but they were from Gt Gransden, and I have found no link to Fenstanton/Woodhurst or Kimbolton.
You need to go back one step at a time - in 1851 they were living in Woodhurst, William born c 1811 in Fenstanton, Sarah c 1819 in Warboys.
Hunts marriage index has a marriage in 1836 at Woodhurst between William Merrill of Colne and Sarah Brown otp, so this should be the place to start. However pre 1837 fathers' names are not shown so apart from names of the two witnesses you may not necessarily learn a lot more than what is shown in the index. But William's parish of residence at his marriage is different to his 1851 birthplace, so I would start with Fen Stanton.
Fen Stanton parish register has been filmed by the LDS and is available for viewing at your nearest LDS Family History Centre. Baptisms have also been extracted onto the BVRI, but the baptisms are not on the IGI at
www.familysearch.org. The bad news is that there is no William Merrill 1806 onwards (or any other Merrill for that matter) in Fen Stanton on the BVRI, so perhaps he wasn't born in Fen Stanton.
In 1861 and 1871 he gave his birthplace as Woodhurst, so that and Colne would be the next parishes to check. Unfortunately the LDS was refused permission to film many parish registers in Hunts, and Woodhurst and Colne are included in this category. Transcriptions are not available from Hunts FHS for these two parishes either. So the parish registers are only available to view at Huntingdon Records Office (who also provide a good research service at very reasonable cost if you can't get to Huntingdon).
In 1841 there was another Merrill family in Woodhurst, William and Elizabeth, who may be connected. There's a marriage on the IGI at Fen Stanton in 1811 which is also on the Hunts Marriage Index - William Myrrill of Woodhurst and Elizabeth Munns who may be the 1841 couple and possibly the parents of your William. But this is speculation and your next step should be the Woodhurst parish register to see if William was baptised there in 1811/12 and to check his marriage in 1836.
Good luck
Regards
David