RootsChat.Com

ok so this is still still a long shot possible but is slightly more possible given the son's name of Alan

As the 1939 register said Joseph Davison was born 1917, I looked for him in 1921 living in Ashington
The 1921 required ages to be recorded in years and months.

Joseph's death registration in 1970 gave a birth date of 21 Mar 1917 and the 1921 census date was taken 19 June 1921, so he'd have been 4 years and 3 months old and there is one entry which matches that. The father on the return is not named Joseph.
BUT this child Joseph is listed as being an 'adopted son' (though they already have a son of that name,so it must have been confusing!)and the record says his mother is dead and the Davison family who had adopted him also had a son called Henry Drysdale  Davison .

Henry Drysdale Davison married Mary F Lee Q1 1931 Morpeth
Harry is often used as an alternate for Henry and Molly for Mary

and searching FreeBMD births 1931-1950 surname Davison MMN Lee in Northumberland

shows Alan's older sister you mentioned born in 1931  and  Alan in Q1 1943 Northumberland Central 10b 567

The 1939 register has Henry D and Mary F listed in Ashington as Davidson, plus one redacted entry which is likely to be their daughter

Boo

Re Joseph D who died c 1974 where in Northumberland did he live?
Nothing obvious for death reg that year in NBL.
Only 1 born 1880

Can you clarify what you mean by "he permanently left England over 50yrs ago"

Unless you have the name "Morton" in your family as a given name or a surname then if is highly likely that the father of your Morton BUSTON had the surname MORTON. Using FindMyPast I see that there were a few families with that surname near Spennymoor in 1891.
To be get evidence of which family had a man or lad who fathered Morton BUSTON you should consider getting a DNA test.
As you may know it was not uncommon for miners families to have a female servant. This occurred mostly where a family had several sons working down the mine but no daughters to help with all the washing, cleaning and cooking. So 9 months before MB was born his mother may have been working as a servant in some village a few miles from Spennymoor such as Coundon or Brandon.