Hi Su and thanks for your help with this. You are right in that I don't yet have Emma and Thomas's marriage certificate. I'm mulling over whether or not to make that investment. This couple are not blood relatives but their daughter was my great grandfather's second wife, and his sister was Thomas's second wife, so the families are a little intertwined !
That 1837 marriage you found is almost certainly the right one. I've just been digging about in the IGI too, and I found an Emma Warburton Whitehead, baptised at Manchester Cathedral on Christmas Day 1840 (ties in with the 6 month old you found in the 1841 census - thanks!), parents John and Mary Ann. Probable siblings include Sarah (1845), Clara (1842), Josiah (1845) Robert Mcreggor (1847), John (1854), Ada (1855), and Edmund (1862).