Hi Ian,
Yes he was

1851 Census
Midma Lodge, Ravens Court, Lag? ( hard to read)
Hammersmith, London
HO107/1469/558/20
Joseph ARESTI, head, mar,42, lithographic artist and surveyor, born Bloomsbury, London
Emma, wife, mar, 40, born Gloustershire, Grate Brech(head?) (sic)
Louisa, daug,unm, 8, born Isle of Wight, Newport
Julia Emma, daug, unm, 6, St Pancras, London
Rosina, daug, unm, St Pancras, London
Trish
EDIT, Did you notice on 1841 Wales census with Sarah and children, that on same page is a James Sargeant aged 45 and wife Margaret. James is not born in county. Possible brother of William?