Firstly, Mary Burn, wife of Robert Burn, was buried 19 February 1800 at Beverley, St John. No age given unfortunately.
I can find a baptism for a Mary Pratman 17 January 1744 in Beverley, father Robert.
There is a baptism 6 April 1734 in Hornsea (about 12 miles from Beverley) for a Robert Burn, parents John and Jane.
I'm not saying that these are your Robert and Mary but worth looking into.