Helen, The reference which Elizabeth gave you from the England civil regt Death register index: Deaths Mar 1865: BREW Samuel Nth Derby 8b 480 looks well worth ordering.
We know that Samuel died between 1861 and 1871, and not in the IOM.

That an age wasn't given in the index is not significant - none were given in these indexes until 1866. The death cert should give you name(s), date, place, age, cause of death, occupation of the deceased, residence if different from the place of death, and the name and address of the informant [which can be crucial information].

The online site which Elizabeth used suggests that you verify their reference from a copy of the index before ordering a certificate. These GRO indexes [sometimes still called St Cath's House indexes] are widely available in FHCs and genealogical libraries.

To order the cert see: http://www.statistics.gov.uk/registration/general_register.asp

Regarding your question about the census entry I listed being confusing, the household was headed by John LENNOX and included his son-in-law Thomas BREW aged 22 b.IoM and Mary BREW his daughter who was married [presumably to Thomas], and his two grandchildren John BREW & Ann E. BREW both aged 2 [presumably the twins of Thomas & Mary BREW].

When you look at the GRO [St Cath's] indexes before ordering Samuel's death cert, it wouldn't take long to also locate the index ref. for the marriage cert of Thomas & Mary. This cert will show, amongst other details, the names and occupations of both fathers, and the witnesses to the marriage who were often relatives.

Of course this may not be John Maxwell Brew's brother, but aren't both certs worth a try?

Sue