Jo,
Do you have the baptisms of their children? At this time the baptism records were very detailed, more so than the marriage records.
5th Oct 1806 is what the IGI has for the baptism of their first child Ann, at Newcastle St. Nicholas. Looking at the Bishops Transcripts its actually 3rd October. Its on the Bishops Transcripts at : Place - Durham, Parish - Newcastle upon Tyne, page 289
Ann Rosenbohm, of Close, born Sept 2nd 1806, baptised October 3rd 1806, 1st daughter of George Rosenbohm, Sugar Boiler, native of Rothenkorn (I think) Hanover, by his wife Catherine, daughter of Geo. Crooks of Gateshead.
The IGI has 3 further children baptised 1808 to 1812 all at Newcastle St. Nicholas, all should be on the Bishops Transcripts. For some strange reason Newcastle St. Nicholas at this time is in the Durham section of the Bishops Transcripts under Newcastle, rather than in the Northumberland section.
Alan.