Many Thanks Again Dobby,
You've given me lots of ideas to chew over...
William James Wilson & Margaret Rees were married at The Parish Church Swansea, after 'Banns'. I have a copy of the Marriage Cert, but I hadn't realized that I could persue the Banns as well...(Would I have to contact the Church to do this, - or are they now on-line somewhere?).
While Margaret Rees's family were long-term residents of Swansea, (I have found them back to 1790 via Parish Records, etc) I can find no trace of William James Wilson there before his marriage. (He doesn't seem to be on the 1891 Census for Wales). However, Swansea at the end of the 19th Century was somewhat of a 'boom town', & I imagine that a Journeyman House Painter, (as William James appears on the Birth Cert of his eldest son, William Havelock Wilson) would have travelled about for work?
It is also quite possible that as William James was born in (approx) 1860, - that his birth was never registered. (However, my Great Grandfather Evan Rees, born Swansea 1838, - was!).
I have tried looking for a William Wilson, (Mason) using the approx dates that you suggest, - but none seem to fit:-( I've also gone through the Censuses searching for William Wilsons/Masons, - but no joy...(Nothing obvious on the IGI either).
William James Wilson himself seems to have been fairly certain that his place of birth was Manchester, & I was also told this by my late Uncle & a cousin...
As I write, I'm looking at his entry for the 1911, in his own handwriting, giving pob as 'Manchester, Lancs.' I can't tell you how frustrating it is not to be able to find him with his family!!

And how I wish that he had an unusual middle name, - especially in view of some of the middle names that he gave his sons

I will take up your suggestion Dobby, & put my Grandfather on the back boiler...while I trawl around again amongst the William Wilsons (no longer) of this world...

Thanks Again, - it is really helpful to look at it anew, through a fresh pair of eyes!
Cheers, Romilly.