Yes. It seems quite possible William or John Morley Markwell might be found as a son of Bartholomew and Sarah.
Many apologies, also, for typing 1778 instead of 1779 for Sarah's baptism
On p. 9 I also found the burial of Thomas Jeffery, presumably her father.
In Horncastle Par/1/7 baptisms and burials 1794 - 1809, I found the baptisms/ christenings of some other children of George and Sarah Rose - p. 51 of 83 was Edwin 1806 p. 53 of 83 was Ann 1808 & p. 56 of 83 was John 1809. By this time our Sarah Jeffery was married to Bartholomew, so perhaps it wasn't her who had the 1st illegitimate child, Rose, or maybe he married another Sarah.
The children I've found so far for Bartholomew and Sarah are:
Bartholomew b 1798 bp 1799 Horncastle
Mary b 1801 bp 1806 Horncastle m 1833 West Ling Wainfleet All Saints
Benjamin b 1803 bp 1804 Horncastle
John bp 1806 Horncastle
I don't have Thomas's baptism or that of Harriet Dickinson his wife, or their marriage, but I have the baptisms of several children to them in Wainfleet All Saints and re Harriet's parents I have John Dicki/enson bp 1789 Hogsthorpe m Mildred Sebright Addlethorpe 1817 and bu 1877 Wainfleet All Saints, with parents Edward Dickinson & Mary. Edward was bu Hogsthorpe 1791. Mildred Sebright was bp 1796 Addlethorpe with parents George Sibright (ch 1753 in Addlethorpe) & Mary Burntoft sp 2 of George Sibwright , who she m 1790 in Huttoft. I have the baptisms of John Dickinson & Mildred Sebright's other children, but not Harriet.
Sarah m 1831 Francis Hoyles? Wainfleet All Saints
Susannah m 1833 Jeremiah Sharp Wainfleet All Saints
Hope this is of help