I know I promised, but things changed, A lot of change. I thought I cracked it, but then a huge curve ball with a few spanners thrown into the works along with any other simile and metaphor you can come up with, came along. This is just a huge headache.
I will not give you my conclusions, as it could influence your thinking, you may think of something different, which you may not have done had I influenced you.
SUMMARY For you to look at.
Thomas HENDRY b abt 1801 in Montrose. - birth details obtained from 1851 census.
Wife: MARGARET (MILNE) HENDRY b abt 1808.
Married : 1828 or 1829.
Parents of 7 children who use HENDRY or HENRY along with decendants. - this are now established facts which you do not have to worry over.
MArriage entry for Thomas and Margaret

I have begun looking at finding about the parents and birth for Thomas. Looked for births for a Thomas HENRY or HENDRY , used 3 search sites, Ancestry, family Search and ScotlandsPeople.
Most likely candidate, and the closest to his declared age in Census is the following entry, is an entry for Montrose Parish records - Thomas HENRY born to Alexander and Jean (FERRIER) on 30th July 1799.
Please note the name is HENRY
Looking for parents marriage raised this interesting Conundrum. I am more than aware that it is possible for two family groups to be nearly identical in name, ages and even giving children same ages born same year. However, there is a little twist to the following.
First entry, we have a record from Montrose, date not shown, but at top of margin on original document, dated 03 FEB 1793which reads:
"Alexander HENRY of this parish & Jean FERRIER of the Parish of Brechin were proclaimed in order to marriage and no objections offered, "

However there is also this entry for BRECHIN.
"29th Jan 1793, Alexander HENDRY, mailman in MONTROSE and Jean FERRIER, servant at Kintrockat (house) in this parish were contracted in order to marriage three several(?) times and no objections raised were married on the 5th day of FEB.

To recap, in case you missed the obvious! We have a parish entry in BRECHIN FOR Alexander
HENRY From MONTROSE and Jean FERRIER from BRECHIN married on 03 FEB 1793
Then we also have a Parish entry in MONTROSE for Alexander
HENDRY From MONTROSE and Jean FERRIER from BRECHIN,married on 05 FEB 1793
IN case you were wondering, the two places are next to each other.

What do you make of this! same names from same places, married 2 days apart in adjoining parishes a stone throw from each other with different surnames of HENRY and HENDRY.