Will certainly let you know if I find out anything. Unfortunately, the IGI does not have the baptisms of either John or Thomas. Wanstead and Clapham do not seem to be on the IGI as far as I can see.
It does, however, list the baptisms of John and Mary's children, and Thomas and Jane's, and the two marriages. Do you know who the witnesses were to the marriage of John and Mary or Thomas and Jane?
There are a lot of variations on the spelling of the surname for John and Mary's children as follows:-
Jennet
Tennett
Jannett
Mackqennet?!!

As you say, loads of different surnames!
Thomas and Jane's children are easier to trace as all seem to be put on under the name Jannett!