Hello Lesley,
The information comes from his death certificate (found via the GRO Marine Deaths Index on findmypast).
My John Ilett (as far as I've been able to work out!) was born 13 Apr 1842 in Stratford, Essex, the son of John Ilett (b. c. 1811 in Stratford) and Rebecca Clark (c. 1811 - 19 Apr 1891).
He married Mary Ann Harness (b. 11 Oct 1858, St Lukes Finsbury) at Holborn St John on 27th Dec 1886.
They had a son, also John Ilett, born c. 1889 in Limehouse.
Typing this out, I realise I've not looked at this family for a while, so must have a look at all the parish records on Ancestry to see if I can fill in any more gaps. Though, as you'll have worked out, it's a difficult surname as it's so odd, it's always mis-spelled on censuses.
I'm descended from John's sister, Sarah Ann Ilott (c. 1839 - 10 Dec 1915), who married William Wilson Palfreman (c. 1836-16 Feb 1904)
I'm intrigued by your John Ilett and his wife though. Both yours and mine have wives called Mary Ann. And your tale of running off, could well be going off to sea. Intriguing ...