I found Thomas very easily in 1861: I just searched for a Thomas (any surname) born ca. 1837 Darlaston with a wife Sarah.
On their marriage certificate his father was Joseph Cockin a lock smith.
Oh well, that sort of destroys my theory.
I'll still post it, just in case:
My starting point was the 1871 census:
1871RG10/2955 42 12
Darlaston, Staffordshire
Bilston StreetEdward Wilkes, head, 38, Nail Maker
Catherine, wife, 44
Thomas Cockin, brother in law, mar, 35, Lock Filer
Richard Cockin, brother in law, mar, 50, Bolt Turner
All born Darlaston
I couldn't find a Thomas with a surname ressembling Cockin in 1841 or 1851 at all, so I searched for a Thomas (any surname) and among the possible candidates, there was one in the same household as a Catherine and a Richard : a Thomas Wood with parents Henry and Nancy.
This rang a bell because I had been looking at the Edward & Catherine Wilkes couple, who are on the censuses together from 1851 on, and the one likely marriage I had found was Edward Wilkes to Catherine Wood in 1848.
Also, it seems that the Richard Cockin of 1871 suddenly turns into Richard Wood in 1881:
http://www.rootschat.com/links/0azn/But then maybe the 1871 census is simply wrong and Thomas shouldn't have been down as brother in law.
I don't suppose the 1860 marriage witnesses give any clues?