Hi Wendy

Have you checked the IGI, there is a William Callister and a Martha Fargher who could possibly be the parents of your John. If you look on Brian Lawson's site for Wills you will see there is a Will that you could obtain of a Martha Callister 1868. On the IGI there is no mention of a John being the child of the above parents, but there is a gap between their marriage and the birth of a Thomas.

I hope this leads you in the right direction, good luck.

Christine