Len, thank you again, think you have cracked it!
So, looks like he was born one month before the marriage so highly likely to be the full brother of Esther. I know in 1841 Census the ages were rounded so anything earlier than this would have been grossly out.
Liz is kindly getting me a copy of the Mothers Will 1845 - wonder if the Deed forgery was something to do with their inheritance?
I have emailed the Museum to ask if they would have any copies of the trial - will keep you posted.
Sandra
ps
The extract you gave from the book stated John and Esther Casement - it was John and Catherine Casement nee Creer.