When I first researched my tree, I got back to my paternal g.g. grandfather, William Martin, born about 1804, and in the two censuses he appeared on, he said that his birth place was either "London, Middlesex", or just "London". Now, on one census William Martin was shown living with his family in the house of Amy Martin, the widow of a Thomas Martin (with Amy as the head of the household), and since William was listed as a "brother-in-law", and his son as "nephew", I assumed that William & Thomas were brothers. However, Thomas only appears on two censuses, the 1841 and 1851, and on the 1841 census he's "of this county" (Kent), and on the 1851 he's apparently born in Plaistow, Kent (a hamlet near Bromley).
However, I subsequently found out that Amy (wife of Thomas) and Mary (wife of William) were sisters, so maybe Thomas and William were not brothers at all ? The problem is that when I find things that fit William, they don't fit Thomas. I've just found a possible Christening record for William in St Andrews, Middlesex, which I think was in Enfield, and there are other family connections there, but unless the whole family uprooted themselves and moved to Plaistow, I can't see the possibility of Thomas and William being brothers. Maybe the surname was just a co-incidence ? William married Mary in Plumstead, Kent and Thomas married Amy in Lee, Kent.
I've asked this a couple of times before and it didn't really go anywhere. If I don't resolve it soon, I think I will lose my marbles !

If you need more information, I posted a lot
here in a previous thread.