Hi Glendaveny
I had a quick look on family search, (IGI). There are lots of Cannells on the Isle of Man, but no Trimnells, or even variations of it. There isn't anthing as recent as 1918 on there either.
Maybe Dorothy's mum was born on the Isle of Man, but can't see any sign of William on there. I have ancestors from the Isle of Man, one branch goes way back, but the other branch I haven't had much luck with as yet.
Good luck with your searching
Rosemary