Thanks for the welcome and reply.
I thought I had her on a Census prior to marriage but mistakenly had her mother, Catherine McEntee and father as William G McKay.
Then received the Birth Certificate to show her mother as Mary McKay with no Fathers details, which has thrown it all.
Interesting the illegitimate part you mention, I wouldn’t have thought of that. She does have him down as William MacKay (not McKay) and a Labourer on her Marriage Cert…
Birth is definitely 1886, I think I may have confused that bit - 1879 was the DOB of her husband William Henderson.
Marriage was 19 Aug 1903 in Stranton Hartlepool. Mistakes again perhaps, her age is 18 (not 17) and William Henderson shows as 22 (not 24)