Dear Gibby1968,
On a web site which I subscribe to I searched Parish Marriages between 1740 and 1780 in the name of Robert Coward and twenty one entries came up. I then checked the details of all these and found only one where the brides name was Agnes, the details were Robert Coward married Agnes Preston on the 9th October 1755 at Pennington near Ulverston. However, this is twenty years before the birth of the two daughters which makes it quite likely that this is not the marriage you are looking for.
As a matter of interest I also found a baptism for a Agnes Preston on the 5th August 1733 in Kendal.
Regards,
fifthwheel.