That's a very, very useful find, Richard, thanks very much!
My initial thoughts were as follows: 1) 'Hector Laing of Dean Bank Lodge' married, perhaps, a 'Miss Stanley' (or a woman whose father was, perhaps, named Stanley?) and had two sons, Hector Stanley and Herbert Stanley (not implausible given some Scottish naming conventions), each of whom married a daughter of Sir Alexander Grant, 1st Bt (wikipedia, at least, states 'a son and two daughters', which, given the second record above, would be 'three'- Agnes, Margaret and Elizabeth; the announcement does say 'elder', implying only two; maybe Agnes died young? At any rate, those two records you found seem fairly comprehensive and- although error can always creep in- seems unlikely to be wrong). Alternatively,
2) 'Hector Laing of Dean Bank Lodge' WAS 'Hector Stanley Laing' and had married previously, with 'Herbert Stanley' the 'youngest son' of that previous marriage, with Hector Stanley, on returning from WWI, then marrying Sir Alexander Grant, 1st Bt's daughter Margaret Norris. Given that Herbert Stanley Laing would presumably be at least of a comparable age to Elizabeth Grant, who based on her siblings might be supposed to have been b. 1890s, this would mean Hector Stanley Laing, if the same person as he of Dean Bank Lodge, and if the father of Herbert Stanley, would probably have been born at the very latest 1870s, making him not inconsiderably older than his wife Margaret Norris Grant.
This site
http://digital.nls.uk/rolls-of-honour/archive/100546133?mode=transcription has a record of a Herbert Stanley Laing, former student at the Royal High School, Edinburgh, as serving in WWI with the Lovat Scouts, Royal Garrison Artillery, and with the R.A.F. Is it possible even official family-sourced records of 'Hector Stanley Laing' were in error, and he was Herbert all along? Perhaps went by 'H. Stanley Laing', leading descendants to misconstrue the initial as his father and son's name, Hector?
EDIT: looking for Elizabeth (Grant) Laing's will post-1927 (which I can't find), I came across the following in 1928:
'LAING, Hector of Deanbank Lodge Deanbank-lane Edinburgh died 2 December 1927 at London Confirmation of Agnes Yule or Laing widow.' Presumably this is the 'elder' Hector Laing (mentioned in your first record above), father of Herbert Stanley (and possibly also Hector Stanley) Laing, as the Hector (Stanley) Laing who married Margaret Norris Grant had a second son, Robert, b. 1929, which would of course not be possible had he died in 1927 (notwithstanding, of course, the presence of the new name 'Agnes Yule or Laing', which clearly indicates he's a different individual) This puts paid pretty solidly to my option 2) above, at least (albeit it was a stretch from the start!) FreeBMD has the Dec 1927 record of the death of Hector Laing at Hampstead, aged 69.
Additionally, on Ancestry under 'public member stories', is a file entitled 'Hector Laing Yule marriage 1881' and attached to 'Agnes Yule (born 1864)'- a cross-check gives it also as attached to 'Hector Laing (1860-1927)- given the age of Hector Laing, this would be a plausible year of birth for his wife, and the shared attachment of the file is strongly suggestive.
A look at familysearch gives a Hector Laing, b. 21 July 1860, Edinburgh, Midlothian, s. of Hector Laing and Mary Liston (several other children also appear in records), and a Hector Laing b. 1860 resident at 'St George, Midlothian)- I had to check (geography not being my strongest suit), but this is a registration district of Edinburgh, which is corroborative I think- in the Scottish Census 1861, 71, 81 and 91.