And thank you Rosie99 for replying too.
"My" Thomas Livermore drowned in the river Esk in VDL on the 14th Sept 1827 aged 40.
I assumed he came from Surrey because he was arraigned at the Croydon Surrey Assizes on the 27th March, 1811. ASSI 35/25/3 PFF4456 shows him as being "late of the parish of Warlingham in the County of Surrey Labourer". But, of course, he didn't have to have been born there.
The marriage date fits and a potential birth date for son James.
To fill you in: There was a claim during the trial that Mary taunted Thomas with not being James's father, which prompted Thomas to try to cut her throat (she survived; and went on to repartner and have more children). I can see that Thomas might have fled towards London, eventually arrested in Surrey.
I don't have a birth certificate for James but apparently he was born "exactly 8 months and 27 days [after the marriage] making it highly unlikely that Mary was pregnant by another man before her marriage".
It would be interesting to trace James's descendants, but especially the descendants of any siblings Thomas may have had. I may be able to start a trace for them via James and Eleanor Livermore.