As an update, I have found information on WorldConnect which leads me to believe that Margaret Borthwick was baptised 2 February 1792, to James Borthwick and Marion Thomson, in Kirkliston, West Lothian.
William Barr was born 14 March 1791, to John Barr and Agnes Akeford, in Ratho, Midlothian.
I have found the birth registrations for both of these.
According to the same WorldConnect site - the marriage ocured 1810 in Kirkliston, West Lothian, Scotland. But I still can't find this marriage rego anyway in SP. Is there something I'm missing?