Hi Trish and Roy
So much to think about... You're all being incredibly helpful and very thought provoking. Thanks

Sorry, misunderstood about there possibly being 2
wives named Elizabeth. Have searched NSW BDMs however and can't find another marriage listed for William in Stockton (or anywhere else NSW for that matter) for 1856. The only other marriage listed in NSW between 1850 and 1865 is for a William Thurgood to Charlotte Hunt in Sydney in 1852. Don't think this is matches the record found on the IGI. I'm looking for parish registers for Stockton, but microfilms of these seem only to cover 1888-1893. Shall put a call out for a lookup of the actual registers if possible.
If my original working assumption that William is the convict who arrived in Oz in 1840 on the "Eden" - the ships list states that this William Thorogood was married with 3 children (1 male 2 female) when he was transported. The other info from this document is that he was aged 25, could read and write, of Protestant religion, his occupation was a black and white smith, from Essex, and tried at the Kent Assizes on 9 March 1840 for sheep stealing, sentenced to 15 years transportation, with no previous convictions, and he was 5' 8 1/2". This would make this chap's birth year abt 1815.
If this is my man, perhaps those first few children were born out of wedlock to Elizabeth Singleton and that upon the death of his wife in England, he felt free to remarry, hence the 1862 NSW marriage to Elizabeth Singleton... What do you think?
I am ordering a couple of NSW certs to see what else has been documented for a clearer picture. As you know, I already have the 1862 Thoroughgood/Singleton marriage, and the birth, marriage and death certs of their daughter Elizabeth Ann.
Roy, very interesting that you have encountered a familial occupation link between the Thoroughgood name and blacksmithing. This might be very useful in discerning between the different Williams contemporary to mine.
As for Elizabeth Singleton, she arrived in Oz onboard the "Briton" in 1844 aged 5 with her parents Richard (ag labourer) and Hannah (farm servant) and siblings William 15, Martha 12, Richard 9, John 7, Samuel 2. Other researchers have linked Elizabeth to a ?great uncle Benjamin Singleton who founded Singleton NSW and is purported to have paid for her family to come to Oz, and ?great grandfather William Singleton who was convicted at the Old Bailey in 1791 for larceny and transported to Oz arriving on the "Pitt" in Feb 1792. So Roy, not the daughter of a felon, but possibly a great granddaughter. That's my next research endeavour

Many thanks again
MM