You would need to use Scotlands People to search for their marriage. It is a pay per view website and images are downloadable up to 1937. A basic search on their free index shows no matches under Pretty or Dunnett between 1917-1930
From the 1901 census - Christina was the daughter of Thomas Dunnett b 1874 Keiss Wick and Christina b 1875 Wick
She had siblings John Miller Dunnett b 1894 Caithness and David b 1899 Dunnet
Dunnet ED 4 Page 11 Line 20
I can't see a Scottish 1901 entry for Samuel and there were no families called Pretty in Scotland in 1901
The English 1901 has 2 possibilities - b 1896 Alderley Edge Cheshire - parents Samuel & Rose
or b 1900 Willesden Green Middlesex parents Henry & Ellen
Sight of their marriage cert will give you his parents names
www.scotlandspeople.gov.ukThere are also others with earlier birthyears in the 1890's