Rosie, the Ancestry tree birthdates are an exact match for my man, but unfortunately I have a sub for FindMyPast not Ancestry, so can't access them! Is there any way of me contacting the owner/s of the trees directly? I'd be over the moon if I can find living descendants, as William was my great-grandfather's brother. I have managed to trace all of the other siblings, barring William and his brother Rankine Forrest MacGregor, but so far haven't been able to dig up any living descendants of those of them who married.
Frustratingly, there was another William Taylor MacGregor living in Edinburgh around the same time as my man: he was born in 1874, Liverpool, and went on to become assistant governor then chaplain of Craiglockhart Poor House in Morningside. He has muddied the waters considerably as all the info I could find related to him.