On the FreeBMD web site ( www.freebmd.org.uk/ ), there is a Dec. 1848 marriage listed in the index for a Sarah Addems and Thomas Mann in Newton Abbot Dist., vol. 10, page 245. Newton Abbot, I believe, might be located in South Devon. Here's the Devon record office web site ( www.devon.gov.uk/record_office.htm ) that has an e-mail contact link if you want to ask about the cost of a marriage record.

Also, in the 1860 Warren Co, IL census, Alice Mann, age 9, the oldest child of Thomas and Sarah Mann, is listed as being born in Illinois, so, you might try checking on Ancestry for 1849-1851 passenger lists for a Thomas Mann. He didn't enter the U.S. through Ellis Island because the immigration station didn't open until 1892.