Hi Paul,
According to John and Katherine's marriage record (23 Nov 1806):
John was the son of William MacKenzie, tenant in Melvaig (Gairloch).
Katherine was the daughter of William Bayne, tenant in Melvaig.
This could be John & Catherine in Melvaig in 1841:
John MacKenzie 58 Tenant
Catherine 53
John 28
Ann 21
Margaret 20
Alexander 15
Murdo 11
Alexander 1
I guess the 1 year old Alexander could be an illegitimate son of one of John's daughters. William is not here, which is fine, because it would match the info you have that he left for Wigan around 1830.
Although I have lots of BMD records for Gairloch, I don't have any that match the family members that appear in 1841.
Cheers,
Kiko