Hi
There might be a possibilty for the marriage of James + Sarah in 1804?
I haven't seen the details, perhaps you can get hold of them, but is indexed thus on findmypast
1804 in Mathern, Monmouthshire
James Gardener + Sarah Howell James

they index her surname as Howell James
source is Gwent FHS (Chepstow branch)

It seems that James Gardiner had a sister Jane, and I can find her in Liverpool (Wapping) in 1841, with Samuel and Sarah. In the 1851 census, Jane Gardiner (it may say Gardin or Gardine?), possibly born Monmouth, is given as aunt to Samuel.
I did look some of this up, in 1861 in Liverpool Samuel Gardiner has two nieces called Martin living with him, who may be children of your Emma? Sarah Gardiner married Charles Blades in 1863, there is an announcement of this in the Liverpool Mercury. "Sarah daughter of the late James Gardiner, Esq., merchant, of this town"
However, there is no point me going through everything here because I now find there is a website devoted to the Gardiners! It has the death certificiate of James died 1837 on there.
This is the James + Sarah section
http://www.gardinerhistory.0catch.com/James%20and%20Sarah.ht...
Obviously I cannot vouch for the accuracy of everything on it. And whoever has created this site appears to have no knowledge of the Gloucestershire connection? Though we can see that James + Sarah were in Newland for a time.
John