Hi Aaron,
If you've read throught the previous posts on this thread, you'll have noticed that some of the records for Polmont burials were lost in a fire.
These records refer to the Churchyard only.
There is also a HUGE Cemetery called Grandsable between Polmont and Grangemouth and it's sometimes called 'Polmont' Cemetery.
The records for this Cemetery are available from the Cemetery Records Office in Camelon, Falkirk. From memory I think they go back to around 1880 but you'd need a date of death in order for them to do a search for you.
Have you managed to find your couple on any Cencus? That would narrow down the dates more and at least give you an indication of when they were still around.
I might have found this couple's children (from the British Vital Records)
GARDINER, Janet
Birth Date: 19 Dec 1790
Christening Date: 20 Dec 1790
Recorded in:Polmont, Stirlingshire, Scotland
Father: James GARDINER
Mother: Jean GARDINER
GARDNER, James
Birth Date: 10 Dec 1792
Christening Date: 30 Dec 1792
Recorded in: Polmont, Stirlingshire, Scotland
Father: James GARDNER
Mother: Jean GARDNER
GARDNER, David
Gender: Male
Birth Date: 4 Apr 1795
Christening Date: 5 Apr 1795
Recorded in: Polmont, Stirlingshire, Scotland
Father: James GARDNER
Mother: Jean GARDNER
GARDNER, William
Birth Date: 9 Feb 1798
Christening Date: 18 Feb 1798
Recorded in: Polmont, Stirlingshire, Scotland
Father: James GARDNER
Mother: Jean GARDNER
GARDNER, Margaret
Birth Date: 20 May 1800
Christening Date: 23 May 1800
Recorded in: Polmont, Stirlingshire, Scotland
Father: James GARDNER
Mother: Jean GARDNER
GARDINER, John
Birth Date: 17 Dec 1801
Christening Date: 20 Dec 1801
Recorded in: Polmont, Stirlingshire, Scotland
Father: James GARDINER
Mother: Jean GARDINER
GARDNER, Jean
Birth Date: 5 Dec 1804
Christening Date: 9 Dec 1804
Recorded in: Polmont, Stirlingshire, Scotland
Father: James GARDNER
Mother: Jean GARDNER
I wouldn't worry too much about the variations of the surname spelling.
Hope this helps,
Anne