Hi again
I see that on the 1851, they have a son Samuel aged 2. This registration is more easy to find than the other children born post-1837 (John Margaret & Henry). I think this is his birth registration:
March q, 1849,Newtown vol 27 page 143
If you were to get this (specifying parents as John and Jane), it would give jane's maiden name which might help to pinpoint the marriage - unless she was a Jones as well

From this census entry, it looks as if the eldest child, Ellis, was born circa 1836 so the marriage is probably just before that date.
Gadget