This may be yet another portrait of Niel: 

Dundee Advertiser 11 May 1882 p. 1

William Smellie Watson (1796-1874) was a decent portraitist based in Edinburgh and I suspect he would not have wished to copy the famous work by Raeburn, who had been a rival of his father George Watson (1767-1837). I have found no listing of this portrait.

And what of this pair of portraits, father and son?:

Dundee Courier 21 December 1872 p. 3