Late to the party
Unlike those in Downing Street, you can turn up late to this party with total impunity

I know that Alexander Ruddach and Mary Ritchie lived in Balnamoon and had two sons called Joseph, one baptised on 24th May 1755 and the other on 8th January 1760.
Witnesses to both baptisms include Joseph Ruddach and Joseph Annand. Joseph Ruddach's father and Joseph Annand's mother were brother and sister. I am 99.9% certain that your Alexander Ruddach is Joseph Ruddach's brother and Joseph Annand's cousin.
There were four cousins called Alexander Ruddach: Alexander (b 1704), son of Alexander R; Alexander (b 1711), son of George R; Alexander (b 1717), son of William R; Alexander (1718-1763, minister of Kirkwall), son of Thomas R. The first two are a bit early. The last one can be ruled out. The one born in 1717 is the only one with a brother called Joseph.
The Ruddachs can be traced back in Grange to the 16th century.