Gillian,
I spent a week using all the online information to make a tree which can be seen on my website.
My goal was to find some relative still living on the IOM who might be able to verify that my ancestor did go to Canada. That didn't work, but I was recently able to prove the IOM connection from records of his daughter's family.

Your Alice Postlethwaite Brew was sister to my g-g-g-grandfather, Richard Williamson Postlethwaite (1797 IOM - 1871 Goderich,ON,Canada). My friend got their father's will (which mentions Alice & John Brew) from Utah records. (It just says they don't get anything, which is too bad because Brew would be the perfect name for a pub proprietor!) If you email me kathleen AT haymountain.com, I'll send it to you.
I may go to IOM in June. Are the tombstones of John Postlethwaite (d.1830) and/or Martha Sawrey Postlethwaite (d. 1803) still visible in the Douglas graveyard? I know the Brown Bobby and the Old Swan Hotel were torn down long ago.
Thanks for posting, Kathleen