HI Pat
Sadly, Old Parish Register entries rarely give any extra information about the bride and groom, other than names. Very occasionally, you might get an extra bit of info (name of father/ parish they were from/ occupation of groom etc) but mostly it's a basic single line entry. It's a lucky dip unfortunately, you won't know what's on the extract until you order it and see.
But there are lots of other avenues for you to follow up.
These are the children showing for John and Margaret Mortimer on IGI:
1. JANET ALLARDYCE MENNIE Birth: 28 AUG 1849/ Christening: 11 SEP 1849 Peterhead, Aberdeen, Scotland
3. ROBERT MENNIE Birth: 07 SEP 1847/ Christening: 13 FEB 1848 Peterhead, Aberdeen, Scotland
5. JAMES MENNIE Christening: 19 FEB 1839 Saint Fergus, Banff, Scotland
6. MARY SLICER MENNIE Christening: 11 DEC 1837 Saint Fergus, Banff, Scotland
As you can see, there are at least two surnames being used as middle names for their children that could help establishing links.
You also have the census route. There is one birth showing in 1849 in Peterhead, might be a good place to start finding the family in the 1851 Census which is better at stating birth places and ages than the 1841 Census.
Also, I don't know if John and Margaret died in Scotland or moved abroad. If they died in Scotland, you should be able to find their death certs on Scotlands People, the official pay to view BMDs and Census site. Their death certs. would tell you the names of their parents, including mothers' maiden names.