Hi Ray

You say that Isabella was probably born 1818, which makes me think that you may not have found Joseph and Isabella Clarkson in the 1841 and 1851 censuses ( or that I've got the wrong family !)

Have a look at 1841 Lancashire / Manchester / Market Street / District 3 / page 4. Joseph is a miller age 28 and Isabella is 29, "not born in Lancs.". Children are Jane 8, Mary 5, Isabella 3 and William 1.

Then in 1851, go to Lancs / Manchester / Market St / District 1c page 14. Isabella is "from IOM " age 39.

Jane is possibly at Lancs / Manchester / Deansgate / District 1j / page 17, and with her is a Jane Kelly from IOM.

In 1861, Joseph could be the widower at Lancs / Openshaw / District 24 / page 20.

It looks as though Isabella may have been born in 1811/2.