Thanks so much. My Mckerlie is also the 1906 edition - I can't get access to the online version (we have tighter copyright laws in the UK) so searching it for other mentions is difficult. I suspect that William Jr is the father of 'my' Margaret because of the dates.
There is a mention of the McDowell/Haltridge marriage in a doc from the Barnbarroch papers, a report on which seems to Patrick McD's "brother-in-law" as John - but it's a bit garbled and I still need to find proof!
There's no way to be sure without some other shred of evidence which will, I hope, turn up one day!