Thanks Barbara & Luzzu.
That looks very probable but also looks like it could be a rather tangled one to solve.
What I have so far now is:
1841 - HO107-573-B3-F37-P27 - Black Lion Court, 6, born in county (Martin Wimbush)
1851 - HO107-2229-F176-P31 - Long Millgate, 16, butcher, b.Warrington(?) (Martin Wimbush)
1856 - Marriage - Anne (Hutchinson or Helme) Q2 Manchester 8d 559 (Martin Knight)
1861 - RG09-2950-F84-P26 - Great Ducie St, 27, pork butcher, b.Bold (Martin Knight)
1868 - Marriage - Mary Isherwood Q2 Manchester 8d 649 (Martin Nathaniel Knight)
1871 - RG10-4044-F158-P16 - Ducie St, 36, butcher, b.Farnsworth (Martin N Knight)
1881 - RG11-3992-F13-P20 - Great Ducie St, 45, pork butcher, b.Farnsworth (Martin N Knight)
1891 - RG12-3265-F83-P37 - Ellesmere St, 55, baker, b.Farnsworth (Martin N Knight)
1901 - RG13-3770-F149-P5 - Ellesmere St, 65, baker, b.Farnsworth (Martin Knight)
1911 - RD467 SD1 ED29 SN120 - Ellesmere St, 75, baker, b.Warrington (Martin Nathan Knight)
He's pretty consistent with his age, and doesn't look like he moved around much at all.
Although FREEBMD doesn't have Martin Knight & Mary Isherwood on the same page, Mary's index entry is clearly 649, Martin's is unclear and looks like 619 (number of grooms doesn't match brides). However, one of Martin and Mary's daughters is Rachel Isherwood Knight on the 1911 census.
Searching the IGI I found:
Martin Nathan KNIGHT bp.28 OCT 1792, Farnworth, parents Peter Knight & Betty, C005732.
That's too much of a coincidence to be unrelated, but obviously can't be the same person.
There is also a Peter, same batch/parents bp.13 MAR 1796, Farnworth.
I can not find any Martin Knight on 1841/51 censuses.
So, as Luzzu suggests, I'll order the cert of one of the Wimbush children in the hope of getting Betty's (Samuel Wimbush's wife) maiden name.
Would it be worth ordering one of Martin's marriage certs to see who he names as his father?