Hi Dave,
I had a wee look at Campsie Churchyard MI's (in the Stirlingshire West book) but although there were a few Forrests, none match up with your folks.
The following are all the ones listed in Campsie:
1.
J C E M 1795
William Calder, his wife Mary Forrest & daughter Jean who died 8th July 1843 aged 15.
2. (Table Stone)
John Brown & Jean Forrest & Heirs
Malcolm Brown, Margaret Winning his wife who died 16th June 1839 aged 21
3. (Table Stone)
William Forrest died 14th July 1853 aged 78
His wife Margaret Frew died at Glorat Mill 8th November 1836 aged 50
4. (Flat Stone)
1773
I F I W
John Forrest, Miller, French Mill & his wife Elizabeth Millar & their children
(Stone has a carving of a miller's rhind)
5. Flat Stone) next to above
John Forrest at French Mill, his wife Margery Wilson
Their 1st son John died 30th May 1822
John Forrest died 24th April 1863 aged 93
6. (Flat Stone)
Charles Forrest in French Mills, his wife Helen Brown & their children. 1842.
7.
1842 Charles Forrest
On the offchance, I had a wee squint at Slamannan & Lennoxtown churchyards but there's no Forrests listed at all.
Neither Cumbernauld or Kirkintilloch are in the books so I couldn't check that out.
One thing you do notice in the older churchyards - there are lots of stones just bearing one or two sets of initials, usually husband and wife but unless it's directly beside another one with matching names in full, there's not really much chance of proving who it is. Also, the letter J was quite often carved looking more like an I - just another wee ploy to keep us on our toes.
Drives you nuts - eh?
Sorry I coudn't be more help.
Anne