Hi Aghadowey
There's a family storey that dad was raised by his "sister" Lizzie Lavery.
Lizzie Creaney had married James Lavery.
Someone on RC very kindly managed to put me in touch with one of Lizzie's grandchildren, Nick, and Nick's mum remember's visiting my dad in Bristol in the 1960's and her husband referred to my dad as his cousin (I think Lizzie was a young aunty rather than his sister).
I also remember a cousin of dad's called Noel Lavery coming to visit us when I was little.
So I know there is a definite connection here.
Nick has told me his grandma Lizzie Creaney was the youngest child in this particular family.
http://www.census.nationalarchives.ie/reels/nai001583372/So I'm now trying to document a link back from Lizzie to check this is the right family.
My feeling is it's possible that my granddad may have been William John in this family, but known to the family as simply John.
Or as you say my dad may have used his granddad's name for his marriage certificate which also fits.
I think I'm close to cracking it, just need a few bits to fall into place.
A birth certificate would just make everything a whole lot simpler.
Checking school registers sounds like a good idea.
But again it depends on me getting across to Proni to do it.
Thanks again for all the suggestions.