Have you found them on the Census ?
Possible matching couple on the 1851 , 1861 , and 1871 Census in Dalkeith(with child Marion ?).
Scotlandspeople has -
Death for an Elizabeth Howatson(or Sim) , age 59 , 1871 Dalkeith , Midlothian.
Death for a James Howatson , age 66 , 1874 Dalkeith Midlothian.
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