I can see Alexander in 1871. There is an Alexander Duthie living in Peterhead, born Peterhead 1850, but he is a fisherman. I can't see him in England, either.
You may already have these:
Birth reg. Jessie Ann Duthrie Q3. 1877 Newcastle on Tyne district, Vol. 10b Pg. 125
Marriage : Alexander Duthie Q4. 1876 Newcastle on Tyne district, Vol 10b. Pg. 154
There are several possible 's for Mary Ann Scott, in the Newcastle area : if you have the marriage certificate this will tell you both father's names - and may make it possible to find them both in 1871 ?
Hope this helps
